Add underscores in manual references.

src/backend/storage/ipc/ipci.c

src/backend/storage/lmgr/multi.c

src/man/alter_table.l

src/man/copy.l

src/man/create_aggregate.l

src/man/create_database.l

src/man/create_function.l

src/man/create_language.l

src/man/create_operator.l

src/man/create_rule.l

src/man/create_sequence.l

src/man/create_table.l

src/man/create_trigger.l

src/man/create_type.l

src/man/create_version.l

src/man/create_view.l

src/man/drop.l

src/man/drop_aggregate.l

src/man/drop_database.l

src/man/drop_function.l