documentation

While the manual mentions FRAC_SECOND only for the TIMESTAMPADD()
function, it was also possible to use FRAC_SECOND with DATE_ADD(),
DATE_SUB() and +/- INTERVAL.

Fixed the parser to match the manual, i.e. using FRAC_SECOND for 
anything other than TIMESTAMPADD()/TIMESTAMPDIFF() now produces a 
syntax error.

Additionally, the patch allows MICROSECOND to be used in TIMESTAMPADD/
TIMESTAMPDIFF and marks FRAC_SECOND as deprecated.