Improve usage details on overriding USER command in Docker run reference page
@@ -1429,7 +1429,10 @@ 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
+ -u="", --user="": Sets the username or UID used and optionally the groupname or GID for the specified command.
+
+ The followings examples are all valid:
+ --user=[ user | user:group | uid | uid:gid | user:gid | uid:group ]
> **Note:** if you pass a numeric uid, it must be in the range of 0-2147483647.
