[roll] Roll fuchsia [gndoc] Update GN build arguments documentation

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1578894
Original-Revision: a6a21284e2f0c6d7c250bd003176a9beb646944e
GitOrigin-RevId: 49cc8291e8449d50a36321fbd3ce8e7451b9b7e2
Change-Id: I3e8746af7d6284318297d2be01af73ca3cd45029

stem

diff --git a/stem b/stem
index 1a6648a..f2af8dd 100644
--- a/stem
+++ b/stem
@@ -8,6 +8,6 @@
             manifest="manifests/platform"
             root=""
             remote="https://fuchsia.googlesource.com/fuchsia"
-            revision="ac0b9e1f00c6ecd21ad43d907f4e147024d4acf6"/>
+            revision="a6a21284e2f0c6d7c250bd003176a9beb646944e"/>
   </imports>
 </manifest>