Document that non-resolvable code owners are ignored

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Ic8e91dfcae42a9dddcf4cee60acdbcb1b817364d
diff --git a/resources/Documentation/backend-find-owners.md b/resources/Documentation/backend-find-owners.md
index 2510e3a..aa7a8b7 100644
--- a/resources/Documentation/backend-find-owners.md
+++ b/resources/Documentation/backend-find-owners.md
@@ -88,9 +88,19 @@
 for consistency an alphabetical order is recommended.
 
 User emails that are added to `OWNERS` files must be resolvable in Gerrit. This
-means, there must be an active Gerrit account that has this email assigned,
-which is only the case if the user logged in at least once into the Gerrit
-WebUI (or if an administrator registered the user programatically).
+means:
+
+* there must be an active Gerrit account that has this email assigned,
+  which is only the case if the user logged in at least once into the Gerrit
+  WebUI (or if an administrator registered the user programatically)
+* the email is not ambiguous (the email belongs to exactly one active Gerrit
+  account)
+* the email has an allowed email domain (see [allowed email domain
+  configuration](config.html#pluginCodeOwnersAllowedEmailDomain)).
+
+##### <a id="nonResolvableCodeOwnersAreIgnored">
+**NOTE:** Non-resolvable code owners in submitted code owner configuration files
+are ignored.
 
 **NOTE:** In Gerrit the visibility of users is controlled via the
 [accounts.visibility](../../../Documentation/config-gerrit.html#accounts.visibility)