bookmarks: update help text since moving into core

Clarifies the help text for the bookmarks command regarding the
requirements for pushing or pulling bookmarks.

mercurial/commands.py

@@ -471,8 +471,9 b' def bookmark(ui, repo, mark=None, rev=No'
     the bookmark is assigned to that revision.
 
     Bookmarks can be pushed and pulled between repositories (see :hg:`help
-    push` and :hg:`help pull`). This requires the bookmark extension to be
-    enabled for both the local and remote repositories.
+    push` and :hg:`help pull`). This requires both the local and remote
+    repositories to support bookmarks. For versions prior to 1.8, this means
+    the bookmarks extension must be enabled.
     '''
     hexfn = ui.debugflag and hex or short
     marks = repo._bookmarks