id: add two examples including the most basic one.

pages/common/id.md

@@ -2,6 +2,10 @@
 
 > Display current user and group identity.
 
+- Display current user id (UID), group id (GID) and groups of which you are a member:
+
+`id`
+
 - Display the current user identity as a number:
 
 `id -u`
@@ -9,3 +13,7 @@
 - Display the current group identity as a number:
 
 `id -g`
+
+- Display an arbitrary user's id (UID), group id (GID) and groups of which they are a member:
+
+`id {{username}}`