ebuild_faq: Add virtual version policy for chromeos-overlay

Previously this overlay did not have an entry and people seem to have
been using `1.5` as a default version for virtuals in chromeos-overlay,
but that conflicts with the version used by "project-" overlays. Add a
new entry to reflect that virtuals in chromeos-overlay should override
those in chromiumos-overlay but not anything else.

BUG=None
TEST=None

Change-Id: I6e733d1a71cfa3fdf274e3fb93b0eac88d469f9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/docs/+/2506132
Tested-by: Chris McDonald <cjmcdonald@chromium.org>
Auto-Submit: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Stephane Belmon <sbelmon@google.com>
Commit-Queue: Stephane Belmon <sbelmon@google.com>
diff --git a/portage/ebuild_faq.md b/portage/ebuild_faq.md
index 37ac06e..fbf43e1 100644
--- a/portage/ebuild_faq.md
+++ b/portage/ebuild_faq.md
@@ -927,6 +927,7 @@
 overlay                               | version (pv)
 ------------------------------------- | ------------
 chromiumos-overlay                    | 1
+chromeos-overlay                      | 1.3
 project-`<project name>`              | 1.5
 project-`<project name>`-private      | 1.7
 chipset-`<chip>`                      | 1.8