Improve some code comments
Improve some code comments

This one at the top, which I added trying to be helpful, is actually
wrong as it confuses base_istream and base_ostream with tls_istream and
tls_ostream. Whoopsie.

Also, add a comment explaining why we do not handle GNUTLS_E_INTERRUPTED
in end_gnutls_io, since "fixing" that "problem" cost me more debugging
time than I care to admit....

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking