Clarify fsmonitor instructions

This change removes references to git versions since Chromium developers
should be using git from depot_tools, and since it isn't completely
clear whether 2.39 is greater or lower than 2.8.

This change also makes it clear that fsmonitor should be enabled on a
per-repo basis rather than globally, for the best cost/benefit ratio.

This change was inspired by the realization that enabling fsmonitor
globally on Windows caused 179 fsmonitor process to be spawned when
"gclient sync" was run. These processes commit an average of 86 MB
on Windows leading to 15 GB of commit (only a few hundred MB of
physical memory is used, but the commit usage is large enough to
matter).

Change-Id: I21c042462022088e11883bbf827276ee585fdaf3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4622567
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1159338}

docs/ios/build_instructions.md

@@ -462,7 +462,7 @@ Or edit the file directly.
 
 #### Configure git to use an untracked cache
 
-If `git --version` reports 2.8 or higher, try running
+Try running
 
 ```shell
 $ git update-index --test-untracked-cache
@@ -475,16 +475,12 @@ If the output ends with `OK`, then the following may also improve performance of
 $ git config core.untrackedCache true
 ```
 
-If `git --version` reports 2.6 or higher, but below 2.8, you can instead run
-
-```shell
-$ git update-index --untracked-cache
-```
-
 #### Configure git to use fsmonitor
 
-If `git --version` reports 2.37 or higher, use fsmonitor, which will
-significantly speed git:
+You can significantly speed up git by using [fsmonitor.](https://github.blog/2022-06-29-improve-git-monorepo-performance-with-a-file-system-monitor/)
+You should enable fsmonitor in large repos, such as Chromium and v8. Enabling
+it globally will launch many processes and probably isn't worthwhile. The
+command to enable fsmonitor in the current repo is:
 
 ```shell
 $ git config core.fsmonitor true

docs/mac_build_instructions.md

@@ -329,7 +329,7 @@ Or edit the file directly.
 
 #### Configure git to use an untracked cache
 
-If `git --version` reports 2.8 or higher, try running
+Try running
 
 ```shell
 $ git update-index --test-untracked-cache
@@ -342,16 +342,12 @@ If the output ends with `OK`, then the following may also improve performance of
 $ git config core.untrackedCache true
 ```
 
-If `git --version` reports 2.6 or higher, but below 2.8, you can instead run
-
-```shell
-$ git update-index --untracked-cache
-```
-
 #### Configure git to use fsmonitor
 
-If `git --version` reports 2.37 or higher, use fsmonitor, which will
-significantly speed git:
+You can significantly speed up git by using [fsmonitor.](https://github.blog/2022-06-29-improve-git-monorepo-performance-with-a-file-system-monitor/)
+You should enable fsmonitor in large repos, such as Chromium and v8. Enabling
+it globally will launch many processes and probably isn't worthwhile. The
+command to enable fsmonitor in the current repo is:
 
 ```shell
 $ git config core.fsmonitor true

docs/windows_build_instructions.md

@@ -492,7 +492,7 @@ fail to attach to some of Chrome's child processes.
 
 #### Configure git to use an untracked cache
 
-If `git --version` reports 2.8 or higher, try running
+Try running
 
 ```shell
 $ git update-index --test-untracked-cache
@@ -505,16 +505,13 @@ If the output ends with `OK`, then the following may also improve performance of
 $ git config core.untrackedCache true
 ```
 
-If `git --version` reports 2.6 or higher, but below 2.8, you can instead run
-
-```shell
-$ git update-index --untracked-cache
-```
-
 #### Configure git to use fsmonitor
 
-If `git --version` reports 2.37 or higher, use fsmonitor, which will
-significantly speed git:
+You can significantly speed up git by using [fsmonitor.](https://github.blog/2022-06-29-improve-git-monorepo-performance-with-a-file-system-monitor/)
+You should enable fsmonitor in large repos, such as Chromium and v8. Enabling
+it globally will launch many processes and consume excess commit/memory and
+probably isn't worthwhile. The command to enable fsmonitor in the current repo
+is:
 
 ```shell
 $ git config core.fsmonitor true