Documentation: update glossary entry for "origin"

Update glossary entry for "origin" to reflect fact that it normally now refers
to a remote repository, not a branch.

Also, warning not to work on remote-tracking branches is no longer necessary
since git doesn't allow that.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/glossary.txt

@@ -188,11 +188,12 @@ octopus::
 	predator.
 
 origin::
-	The default upstream tracking branch. Most projects have at
+	The default upstream repository. Most projects have at
 	least one upstream project which they track. By default
 	'origin' is used for that purpose.  New upstream updates
-	will be fetched into this branch; you should never commit
-	to it yourself.
+	will be fetched into remote tracking branches named
+	origin/name-of-upstream-branch, which you can see using
+	"git branch -r".
 
 pack::
 	A set of objects which have been compressed into one file (to save