[PATCH 0 of 2 ] acl: use of "!" prefix in user or group names (now with user docs)

[elifarley at gmail.com]

acl: use of "!" prefix in user or group names

The "!" prefix allows you to prevent anyone except a given user or group
to push changesets in a given branch or path.

This patch enables a use case suggested by a user (Julien Bonnet):
There's a branch that only a given user (or group) should be able to push
to, and you don't want to restrict access to any other branch that may be
created.

With the "!" prefix, you simply deny access to "!givenuser" (or "!@givengroup").