Need to document the fact that views can't be the source data sets of MERGE statements

The work on DERBY-6652 now prevents views from being used as the source data sets of MERGE statements. We should adjust the Reference Guide section on the MERGE statement accordingly. This statement in rrefsqljmerge.dita:

"sourceTable must identify a base table, a view, or a table function, and it may not be a synonym"

should be changed to

"sourceTable must identify a base table or a table function, and it may not be a synonym"