doc: id: make --help and .man more descriptive

* src/id.c (usage): Improve description, based on a suggestion
from Brian M. Carlson in http://bugs.debian.org/514675
* man/id.x: Use a better one-liner, based on the one at top of id.c.

THANKS

@@ -82,6 +82,7 @@ Bob Proulx                          rwp@fc.hp.com
 Branden Robinson                    branden@necrotic.deadbeast.net
 Brendan O'Dea                       bod@compusol.com.au
 Brian Kimball                       bfk@footbag.org
+Brian M. Carlson                    sandals@crustytoothpaste.ath.cx
 Brian Silverman                     bsilverman@conceptxdesign.com
 Brian Youmans                       3diff@gnu.org
 Bruce Korb                          bkorb@veritas.com

man/id.x

@@ -1,4 +1,4 @@
 [NAME]
-id \- print user identity
+id \- print real and effective user and group IDs
 [DESCRIPTION]
 .\" Add any additional description here

src/id.c

@@ -81,7 +81,8 @@ usage (int status)
     {
       printf (_("Usage: %s [OPTION]... [USERNAME]\n"), program_name);
       fputs (_("\
-Print information for USERNAME, or the current user.\n\
+Print user and group information for the specified USERNAME,\n\
+or (when USERNAME omitted) for the current user.\n\
 \n\
   -a              ignore, for compatibility with other versions\n\
   -Z, --context   print only the security context of the current user\n\