Doc: Clarify purpose of listed build options in DevSetup.md

This file lists a number of commonly overriden build options for
development or special platforms, but was mistakenly taken as a set of
"recommended" options.

Bug: angleproject:1944
Change-Id: Ia50b45724797d5ab071936997b485f9f80a72704
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3234079
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

doc/DevSetup.md

@@ -59,13 +59,16 @@ gn gen out/Debug
 
 On Windows only, ensure you **set `DEPOT_TOOLS_WIN_TOOLCHAIN=0` in your environment** (if you are not a Googler).
 
-GN will generate ninja files. To change the default build options run `gn args out/Debug`.  Some commonly used options are:
+GN will generate ninja files. The default build options build ANGLE with clang and in release mode.
+Often, the default options are the desired ones, but they can be changed by running
+`gn args out/Debug`. Some options that are commonly overriden for development are:
+
 ```
-is_component_build = false      (links dependencies into the build targets)
+is_component_build = false         (links dependencies into the build targets)
-target_cpu = "x86"              (default is "x64")
+target_cpu = "x86"                 (default is "x64")
-is_clang = false                (to use system default compiler instead of clang)
+is_clang = false (NOT RECOMMENDED) (to use system default compiler instead of clang)
-is_debug = false                (for release builds. is_debug = true is the default)
+is_debug = false                   (for release builds. is_debug = true is the default)
-angle_assert_always_on = true   (enable release asserts and debug layers)
+angle_assert_always_on = true      (enable release asserts and debug layers)
 ```
 
 For a release build run `gn args out/Release` and set `is_debug = false`.