Explicitly forbid non-top-level WAIT FOR execution

Explicitly forbid non-top-level WAIT FOR execution

Previously we were relying on a snapshot-based check to detect invalid
execution contexts.  However, when WAIT FOR is wrapped into a stored
procedure or a DO block, it could pass this check, causing an error
elsewhere.

This commit implements an explicit isTopLevel check to reject WAIT FOR
when called from within a function, procedure, or DO block.  The
isTopLevel check catches these cases early with a clear error message,
matching the pattern used by other utility commands like VACUUM and
REINDEX.  The snapshot check is retained for the remaining case:
top-level execution within a transaction block using an isolation level
higher than READ COMMITTED.

Also adds tests for WAIT FOR LSN wrapped in a procedure and DO block,
complementing the existing test that uses a function wrapper.  Relevant
documentation paragraph is also added.

Reported-by: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Discussion: https://postgr.es/m/CAHg%2BQDcN-n3NUqgRtj%3DBQb9fFQmH8-DeEROCr%3DPDbw_BBRKOYA%40mail.gmail.com
Author: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>