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

docs/mac_build_instructions.md

Index: docs/mac_build_instructions.md
diff --git a/docs/mac_build_instructions.md b/docs/mac_build_instructions.md
index 646b64b6332017f78509611280249beedaf4979b..2a81337ea8bfdb8388a8f7b97e323cc1bdd6b8a4 100644
--- a/docs/mac_build_instructions.md
+++ b/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 is `OK`, then the following may also improve performance of

[Sidney San Martín, 2017/05/12 22:50:02]

nit: is ➞ "ends with"

+`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