[docs] clarify support level of goma for win cross

Change-Id: I750c4bd9c159957d7b2f0cfff3d2f6118599f7a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444615
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813154}

docs/win_cross.md

@@ -82,6 +82,10 @@ Add `target_os = "win"` to your args.gn.  Then just build, e.g.
 
 ## Goma
 
+*** note
+**Warning:** This is unsupported and known to not work at times.
+***
+
 For now, one needs to use the rbe backend, not the borg backend
 (default for Googlers).
 Use cloud backend instead.