[Docs] Update the Mac and iOS docs for git status perf improvements.

BUG=none
TEST=none
R=sdy@chromium.org

Review-Url: https://codereview.chromium.org/2876983004
Cr-Commit-Position: refs/heads/master@{#471477}

docs/ios/build_instructions.md

@@ -260,6 +260,8 @@ is open before checking out. This will increase your chances of success.
 
 ### Improving performance of `git status`
 
+#### Increase the vnode cache size
+
 `git status` is used frequently to determine the status of your checkout.  Due
 to the large number of files in Chromium's checkout, `git status` performance
 can be quite variable.  Increasing the system's vnode cache appears to help.
@@ -286,8 +288,22 @@ $ echo kern.maxvnodes=$((512*1024)) | sudo tee -a /etc/sysctl.conf
 
 Or edit the file directly.
 
-If `git --version` reports 2.6 or higher, the following may also improve
-performance of `git status`:
+#### Configure git to use an untracked cache
+
+If `git --version` reports 2.8 or higher, try running
+
+```shell
+$ git update-index --test-untracked-cache
+```
+
+If the output ends with `OK`, then the following may also improve performance of
+`git status`:
+
+```shell
+$ 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

docs/mac_build_instructions.md

@@ -231,6 +231,8 @@ tree is open before checking out. This will increase your chances of success.
 
 ### Improving performance of `git status`
 
+#### Increase the vnode cache size
+
 `git status` is used frequently to determine the status of your checkout.  Due
 to the large number of files in Chromium's checkout, `git status` performance
 can be quite variable.  Increasing the system's vnode cache appears to help. By
@@ -257,8 +259,22 @@ $ echo kern.maxvnodes=$((512*1024)) | sudo tee -a /etc/sysctl.conf
 
 Or edit the file directly.
 
-If `git --version` reports 2.6 or higher, the following may also improve
-performance of `git status`:
+#### Configure git to use an untracked cache
+
+If `git --version` reports 2.8 or higher, try running
+
+```shell
+$ git update-index --test-untracked-cache
+```
+
+If the output ends with `OK`, then the following may also improve performance of
+`git status`:
+
+```shell
+$ 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