Document failure modes of xcb_connect*() functions

Document failure modes of xcb_connect*() functions

Documentation was previously unclear that these always return a non-NULL
pointer, and that callers need to check it for error values, instead of
checking for a NULL return value.

Triggered by having to dig through code to answer a user's question on
the #xcb irc channel, since neither of us found it covered in the docs.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Uli Schlachter <psychon@znc.in>