Document timeLimit attribute on a query

The time limit for a query comes from (in priority order):

1. The value of the timeLimit attribute on the query i.e. <myQuery timeLimit="2000">...</myQuery>
2. The value of the _timeLimit element on the query i.e. <myQuery><_timeLimit>2000<_timeLimit>...</myQuery>
3. The value of the timeLimit attribute in the package definition for the query \
4. The value of the timeLimit workunit option i.e. #option('timeLimit', 2000) in the ECL code, or set via -f on the eclcc command line 5. The default timelimit set for the query in the roxie topology. There are separate default values for low, high and sla priority queries.