Remove the -A option of wbinfo, leaving only the long version,

Remove the -A option of wbinfo, leaving only the long version,
--set-auth-user.  There was enough confusion, as in bug #158,
when a user accidentally typed -A instead of -a, and would get themselves
stuck with a non-working winbind.

I've made the changes to docs/docbook/manpages/wbinfo.xml, but I'm not
sure what to do beyond that.  Is checking that in enough?
(This used to be commit 70fe85e4698ce2c26372b050417986bf4a96680b)