Cleanup of UNION ALL fix. Manual page updates.

src/backend/optimizer/plan/planner.c

src/backend/optimizer/prep/prepunion.c

src/backend/parser/analyze.c

src/backend/parser/parse_clause.c

src/bin/psql/psqlHelp.h

src/include/optimizer/prep.h

src/include/parser/parse_clause.h

src/man/select.l