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.


mysql-test/r/func_time.result:
  Added a test case for bug #33834.
mysql-test/t/func_time.test:
  Added a test case for bug #33834.
sql/sql_yacc.yy:
  Reject FRAC_SECOND for anything other than TIMESTAMPADD() or
  TIMESTAMPDIFF().
  Allow MICROSECOND to be used with TIMESTAMPADD()/TIMESTAMPDIFF().
  Warn about FRAC_SECOND being a deprecated unit.