Tweak //docs/building_old_revisions to mention branch heads.

A contributor helpfully noted that you may need to run
`gclient sync --with_branch_heads` when trying to build
an old revision, so this updates the doc.

Change-Id: I64f70f2fa756187f8b1d57802aa1ed1e6bdb29f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2660155
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#848831}

docs/building_old_revisions.md

@@ -47,6 +47,15 @@ required:
 $ gclient sync -D --force --reset
 ```
 
+Note that if you are attempting to build an old revision that is on a branch,
+you will need to use:
+
+```shell
+$ gclient sync -D --force --reset --with_branch_heads
+```
+
+instead.
+
 **Warning: `gclient sync` may overwrite the URL of your `origin` remote** if it
 encounters problems. You'll notice this when Git starts thinking everything is
 "untracked" or "deleted". If this happens, fix and fetch the remote before