Remove useless comment

Mail priority is documented in config_defaults_inc.php, no point in
duplicating the info here.
WIP: documentation for user signup misleading

config_default l. 345 says signup mails sent even if notifications OFF,
but in fact it's not true

TODO:

- Issues to review / consider fixing:

http://www.mantisbt.org/bugs/view.php?id=6464
http://www.mantisbt.org/bugs/view.php?id=8528
http://www.mantisbt.org/bugs/view.php?id=9295
http://www.mantisbt.org/bugs/view.php?id=13355

- Documentation update