Improve privilege documentation for maintenance commands.

Improve privilege documentation for maintenance commands.

The documentation of the required privileges for maintenance
commands (i.e., VACUUM, ANALYZE, CLUSTER, LOCK TABLE, REFRESH
MATERIALIZED VIEW, and REINDEX) is redundant, inaccurate, and
difficult to read.  This commit fixes and simplifies this
documentation by removing references to ownership, superuser, and
the pg_maintain role.  In addition, this removes notes about
database-wide VACUUM and ANALYZE, clarifies matters for REINDEX on
partitioned indexes and tables, and strengthens the description of
the pg_maintain role.

Reviewed-by: Michael Paquier, Jeff Davis
Discussion: https://postgr.es/m/20230615041044.GA736001%40nathanxps13