docs: try to give move precise commands in Xcode-Ninja Hybrid section

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 b09951682b826262f9861ee932a2f98c6b41ec52..c8ef5ae29acf0d8771e5599b4433e3065e102bb0 100644
--- a/docs/mac_build_instructions.md
+++ b/docs/mac_build_instructions.md
@@ -143,16 +143,39 @@ With hybrid builds, compilation is still handled by ninja, and can be run by the
 command line (e.g. ninja -C out/Debug chrome) or by choosing the chrome target
 in the hybrid workspace and choosing build.
 
-To use Xcode-Ninja Hybrid, set `GYP_GENERATORS=ninja,xcode-ninja`.
+To use Xcode-Ninja Hybrid, set GYP\_GENERATORS like the following:
+
+```shell
+export GYP_GENERATORS="ninja,xcode-ninja"
+```
 
 Due to the way Xcode parses ninja output paths, it's also necessary to change
 the main gyp location to anything two directories deep. Otherwise Xcode build
-output will not be clickable. Adding
-`xcode_ninja_main_gyp=src/build/ninja/all.ninja.gyp` to your
-`GYP_GENERATOR_FLAGS` will fix this.
+output will not be clickable.
+To make this change permanent, you can edit `chromium.gyp_env` (or create it if
+it does not exists) and define GYP\_GENERATOR\_FLAGS. In general, to use hybrid
+mode, your `chromium.gyp_env` could contain the following:
+
+```json
+{
+  "GYP_GENERATORS"     : "ninja,xcode-ninja",
+  "GYP_GENERATOR_FLAGS":
+      "xcode_project_version=3.2 " +
+      "xcode_ninja_main_gyp=src/build/ninja/all.ninja.gyp",
+}
+```
+
+After, generate the project files with:
 
-After generating the project files with gclient runhooks, open
-`src/build/ninja/all.ninja.xcworkspace`.
+```shell
+gclient runhooks
+```
+
+And finally, open it:
+
+```shell
+open build/ninja/all.ninja.xcworkspace
+```
 
 You may run into a problem where http://YES is opened as a new tab every time
 you launch Chrome. To fix this, open the scheme editor for the Run scheme,