Small documentation update regarding debugging aids: - doc/debugging.txt: removed HYPERDBDEBUG and HYPERDBTRACE and replaced it with the text for SENDMAILDEBUG from doc/developers.txt. A short code inspection showed that to be correct. - refered to debugging.txt from developers.txt to have one unique place for documenting debugging aids. - Clarified for SENDMAILDEBUG that no email will be send if it is set.