ios docs: Mention other possible .setup-gn location

The build instructions only mentioned $HOME/.setup-gn, but it is also
possible to have per-checkout .setup-gn files.

Change-Id: I9e446c845761031ede39d3675cb2934795ed5eaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6088418
Reviewed-by: Justin Cohen <[email protected]>
Commit-Queue: Raphael Kubo da Costa <[email protected]>
Auto-Submit: Raphael Kubo da Costa <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1394952}
diff --git a/docs/ios/build_instructions.md b/docs/ios/build_instructions.md
index c58f6ed..2bf7b518 100644
--- a/docs/ios/build_instructions.md
+++ b/docs/ios/build_instructions.md
@@ -83,9 +83,15 @@
 More information about [developing with Xcode](xcode_tips.md). *Xcode project
 is an artifact, any changes made in the project itself will be ignored.*
 
-You can customize the build by editing the file `$HOME/.setup-gn` (create it if
-it does not exist).  Look at `src/ios/build/tools/setup-gn.config` for
-available configuration options.
+You can customize the build by editing a file called `.setup-gn` (create it if
+it does not exist). It can be stored in two locations:
+
+* `$HOME/.setup-gn` (the settings will be applied to all Chromium checkouts).
+* The directory above `src/` (i.e. the directory containing your `.gclient`)
+  for checkout-specific settings.
+
+Look at `src/ios/build/tools/setup-gn.config` for available configuration
+options.
 
 From this point, you can either build from Xcode or from the command line using
 `autoninja`. `setup-gn.py` creates sub-directories named