bookmarks: Improve documentation

hgext/bookmarks.py

@@ -67,10 +67,10 b' def bookmark(ui, repo, mark=None, rev=No'
     deleted. It is possible to use bookmark names in 'hg merge' and 'hg
     update' to update to a given bookmark.
 
-    You can use 'hg bookmark [NAME]' to set a bookmark on the current tip
-    with the given name. If you specify a second [NAME] the bookmark is
-    set to the bookmark that has that name. You can also pass revision
-    identifiers to set bookmarks too.
+    You can use 'hg bookmark NAME' to set a bookmark on the current
+    tip with the given name. If you specify a revision using -r REV
+    (where REV may be an existing bookmark), the bookmark is set to
+    that revision.
     '''
     hexfn = ui.debugflag and hex or short
     marks = parse(repo)