Update win_cross goma directions

Remove reference to needing to switch to the RBE backend now that it is
default, and add current needed workaround.

Change-Id: I52bcf7301c87f30258babba91ddbbff574dbd77d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612218
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840882}

docs/win_cross.md

@@ -86,18 +86,11 @@ Add `target_os = "win"` to your args.gn.  Then just build, e.g.
 **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.
+This should be supported by the default (rbe) backend. However, there may be
+issues with arbitrary toolchain support on Linux. This can be disabled via:
 
 ```shell
-    goma_auth.py login
-
-    # GOMA_* are needed for Googlers only
-    export GOMA_SERVER_HOST=goma.chromium.org
-    export GOMA_RPC_EXTRA_PARAMS=?rbe
-
-    goma_ctl.py ensure_start
+GOMA_ARBITRARY_TOOLCHAIN_SUPPORT=false goma_ctl restart
 ```