commands: explain that "hg serve" does not do authentication

mercurial/commands.py

@@ -2882,6 +2882,12 b' def serve(ui, repo, **opts):'
     recommended to use a real web server to serve a repository for
     longer periods of time.
 
+    Please note that the server does not implement access control.
+    This means that, by default, anybody can read from the server and
+    nobody can write to it by default. Set the ``web.allow_push``
+    option to ``*`` to allow everybody to push to the server. You
+    should use a real web server if you need to authenticate users.
+
     By default, the server logs accesses to stdout and errors to
     stderr. Use the -A/--accesslog and -E/--errorlog options to log to
     files.