Documentation: clarify branch creation

Documentation: clarify branch creation

The documentation seems to assume that the starting point for a new
branch is the tip of an existing (ordinary) branch, but that is not
the most common case.  More often, "git branch" is used to begin
a branch from a remote-tracking branch, a tag, or an interesting
commit (e.g. origin/pu^2).  Clarify the language so it can apply
to these cases.  Thanks to Sean Estabrooks for the wording.

Also add a pointer to the user's manual for the bewildered.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>