Merge pull request #15510 from kblin/issue-14795

reference/run: Clarify the use of numeric UIDs

docs/reference/run.md

@@ -1142,14 +1142,17 @@ 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:
+`root` (id = 0) is the default user within a container. The image developer can
+create additional users. Those users are accessible by name.  When passing a numeric
+ID, the user does not have to exist in the container.
+
+The developer can set a default user to run the first process with the
+Dockerfile `USER` instruction. When starting a container, the operator can override
+the `USER` instruction by passing the `-u` option.
 
     -u="": Username or UID
 
-> **Note:** if you pass numeric uid, it must be in range 0-2147483647.
+> **Note:** if you pass a numeric uid, it must be in the range of 0-2147483647.
 
 ### WORKDIR