Our docs about --enable/--disable-visibility

[Paolo Carlini]

.. oh, about *undecorated* namespaces + -fvisibility=hidden, I don't 
have much personal experience, to be honest. My point was mostly that 
providing to the users the option to not have the namespace decorations 
in the first place seems a very sane thing which we should keep on 
supporting in a well documented way (if then they are using -fvisibility 
and attributes I would suppose they know what they are doing!)

Paolo.