add docs on uid mapping

source/authentication/overview/map-user.rst

@@ -19,6 +19,11 @@ Since 2.0 you should use the simpler and faster :ref:`user_map_match <ood-portal
 
 Both with variations will be discussed here.
 
+.. tip::
+
+  Since 4.0 user mapping also accepts UIDs to be returned as well as usernames.
+  This can be helpful at centers where duplicate usernames and/or have multiple
+  domains.
 
 Remote User
 -----------

source/release-notes/v4.0-release-notes.rst

@@ -15,6 +15,7 @@ New Features
 - `noVNC quality and compression defaults`_
 - `Batch connect sessions poll delay`_
 - `System Status application`_
+- `User mapping now accepts UIDs`
 
 Thanks!
 -------
@@ -159,3 +160,10 @@ update the page. The default is 10 seconds. See :ref:`the documentation on statu
 for more details.
 
 Visit :ref:`disabling_applications` to disable this application.
+
+User mapping now accepts UIDs
+.............................
+
+User mapping scripts can now return a UID instead of a username.
+This can be helpful for centers that have multiple domains and possible
+username collisions.