contributing.md: document CL footers

This change migrates the Chromium browser's primary documentation for
CL footers from the Chromium site:
https://dev.chromium.org/developers/contributing-code/-bug-syntax
into the in-tree contributing guide.

There are two bugs to correct the language used in this change:
* https://crbug.com/1108213
* https://crbug.com/1108214

Test: Gitiles preview
Change-Id: I3c67aa008c3d77edc373531a8a698e3af033be0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311826
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Tal Pressman <talp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790739}

docs/contributing.md

@@ -161,9 +161,9 @@ Bug: 123456
 
 A short subject and a blank line after the subject are crucial: `git` uses this
 as a heuristic for tools like `git log --oneline`. Use the bug number from the
-[issue tracker][crbug] (see more on [CL footer syntax][cl-footer-syntax]). Also
-see [How to Write a Git Commit Message][good-git-commit-message], which has more
-in-depth tips for writing a good commit description.
+[issue tracker][crbug] (see more on [CL footer syntax](#cl-footer-reference)).
+Also see [How to Write a Git Commit Message][good-git-commit-message], which
+has more in-depth tips for writing a good commit description.
 
 ### Chromium-specific description tips
 
@@ -376,12 +376,90 @@ If you would like to be notified about changes to a set of files covering a
 topic or an area of Chromium, you may use the [watchlists][watchlist-doc]
 feature in order to receive email notifications.
 
+## Appendix: CL footer reference {#cl-footer-reference}
+
+Chromium stores a lot of information in footers at the bottom of commit
+messages. With the exception of `R=`, these footers are only valid in the
+last paragraph of a commit message; any footers separated from the last
+line of the message by whitespace or non-footer lines will be ignored.
+This includes everything from the unique `Change-Id` which identifies a
+Gerrit change, to more useful metadata like bugs the change helps fix,
+trybots which should be run to test the change, and more. This section
+includes a listing of well-known footers, their meanings, and their
+formats.
+
+* **Bug:**
+  * A comma-separated list of bug references.
+  * A bug reference
+    * can be a bare number, e.g. `Bug: 123456`, or
+    * can specify a project and a number, e.g. `Bug: skia:1234`.
+  * On chromium-review, the default project is assumed to be `chromium`,
+    so all bugs in non-chromium projects on bugs.chromium.org should be
+    qualified by their project name.
+  * The Google-internal issue tracker is accessible by using the `b:`
+    project prefix.
+* **Fixed:** The same as `Bug:`, but will automatically close the
+  bug(s) as fixed when the CL lands.
+* **R=**
+  * This footer is _deprecated_ in the Chromium project; it was
+    deprecated when code review migrated to Gerrit. Instead, use
+    `-r foo@example.com` when running `git cl upload`.
+  * A comma-separated list of reviewer email addresses (e.g.
+    foo@example.com, bar@example.com).
+* **Tbr:** The same format as the `R` footer, but indicates to the
+  commit queue that it can skip checking that all files in the change
+  have been approved by their respective `OWNERS`.
+* **Cq-Include-Trybots:**
+  * A comma-separated list of trybots which should be triggered and
+    checked by the CQ in addition to the normal set.
+  * Trybots are indicated in `master:builder` format (e.g.
+    `tryserver.chromium.linux:linux_asan_experimental`).
+* **No-Presubmit:**
+  * If present, the value should always be the string `true`.
+  * Indicates to the CQ that it should not run presubmit checks on the CL.
+  * Used primarily on automated reverts.
+* **No-Try:**
+  * If present, the value should always be the string `true`.
+  * Indicates to the CQ that it should not start or check the results of
+    any tryjobs.
+  * Used primarily on automated reverts.
+* **No-Tree-Checks:**
+  * If present, the value should always be the string `true`.
+  * Indicates to the CQ that it should ignore the tree status and submit
+    the change even to a closed tree.
+  * Used primarily on automated reverts.
+* **Test:**
+  * A freeform description of manual testing performed on the change.
+  * Not necessary if all testing is covered by trybots.
+* **Reviewed-by:**
+  * Automatically added by Gerrit when a change is submitted.
+  * Lists the names and email addresses of the people who approved
+    (set the `Code-Review` label on) the change prior to submission.
+* **Reviewed-on:**
+  * Automatically added by Gerrit when a change is submitted.
+  * Links back to the code review page for easy access to comment and
+    patch set history.
+* **Change-Id:**
+  * Automatically added by `git cl upload`.
+  * A unique ID that helps Gerrit keep track of commits that are part of
+    the same code review.
+* **Cr-Commit-Position:**
+  * Automatically added by the git-numberer Gerrit plugin when a change
+    is submitted.
+  * This is of the format `fully/qualified/ref@{#123456}` and gives both
+    the branch name and "sequence number" along that branch.
+  * This approximates an SVN-style monotonically increasing revision
+    number.
+* **Cr-Branched-From:**
+  * Automatically added by the git-numberer Gerrit plugin on changes
+    which are submitted to non-master branches.
+  * Aids those reading a non-master branch history in finding when a
+    given commit diverged from master.
 
 [//]: # (the reference link section should be alphabetically sorted)
 [checkout-and-build]: https://chromium.googlesource.com/chromium/src/+/master/docs/#checking-out-and-building
 [chrome-dd-review-process]: http://go/chrome-dd-review-process
 [chromium-design-docs]: https://groups.google.com/a/chromium.org/forum/#!forum/chromium-design-docs
-[cl-footer-syntax]: https://dev.chromium.org/developers/contributing-code/-bug-syntax
 [code-reviews-owners]: code_reviews.md#OWNERS-files
 [code-reviews]: code_reviews.md
 [commit-checklist]: commit_checklist.md