update: do not use the term 'update' when mentioning reverting one file

and give a more precise hint for how to revert such a file

I'm using the term 'revision' instead of 'changeset' in this change to be
consistent with the REV we use in the synopsis.

mercurial/commands.py

@@ -5082,8 +5082,8 b' def update(ui, repo, node=None, rev=None'
     Use null as the changeset to remove the working directory (like
     :hg:`clone -U`).
 
-    If you want to update just one file to an older changeset, use
-    :hg:`revert`.
+    If you want to revert just one file to an older revision, use
+    :hg:`revert [-r REV] NAME`.
 
     See :hg:`help dates` for a list of formats valid for -d/--date.