build: improve rtc_include_tests documentation

"including" them in the standalone checkout is ambigious,
this might mean not checking them out from git.

BUG=None
NOTRY=true

Change-Id: I53334a718c5535f63faa87d9e0fe6575012e0d2f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/209223
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#33388}
diff --git a/webrtc.gni b/webrtc.gni
index c1227ba..b70cba6 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -264,7 +264,8 @@
     rtc_enable_avx2 = false
   }
 
-  # Include tests in standalone checkout.
+  # Set this to true to build the unit tests.
+  # Disabled when building with Chromium or Mozilla.
   rtc_include_tests = !build_with_chromium && !build_with_mozilla
 
   # Set this to false to skip building code that also requires X11 extensions