update ALTER TABLE with ATTACH PARTITION lock mode

commit #898e5e32 (Allow ATTACH PARTITION with only ShareUpdateExclusiveLock)
updates ddl.sgml but not alter_table.sgml, which only says:

https://www.postgresql.org/docs/12/release-12.html
|An ACCESS EXCLUSIVE lock is held unless explicitly noted.

Find attached patch, which also improve language in several related places.

"Without such a constraint": SUCH could refer to either of the constraints..

"because it is no longer necessary.": In our use case, we prefer to keep the
redundant constraint, to avoid having to add it back if we detach/reattach
again in the future..