Rename postgres95 to PostgreSQL. Add comment for SELECT NULL

src/backend/executor/execTuples.c

src/man/abort.l

src/man/alter_table.l

src/man/begin.l

src/man/built-in.3

src/man/catalogs.3

src/man/cleardbdir.1

src/man/close.l

src/man/cluster.l

src/man/commit.l

src/man/copy.l

src/man/create_aggregate.l

src/man/create_database.l

src/man/create_function.l

src/man/create_index.l

src/man/create_operator.l

src/man/create_rule.l

src/man/create_table.l

src/man/create_type.l

src/man/create_version.l