reference/run: Clarify the use of numeric UIDs

it is possible to pass an UID that has not been created inside the container, clarify this in the docs.
This should fix issue #14795

Signed-off-by: Kai Blin <kai@samba.org>

docs/reference/run.md

@@ -1142,10 +1142,11 @@ volume mounted on the host).
 
 ### USER
 
-The default user within a container is `root` (id = 0), but if the
-developer created additional users, those are accessible too. The
-developer can set a default user to run the first process with the
-Dockerfile `USER` instruction, but the operator can override it:
+The default user within a container is `root` (id = 0), but if the developer
+created additional users, those are accessible by name.  When passing a numeric
+ID, the user doesn't have to exist in the container. The developer can set a
+default user to run the first process with the Dockerfile `USER` instruction,
+but the operator can override it:
 
     -u="": Username or UID