doc: Document that MAINTAIN privilege allows statistics manipulation functions.

doc: Document that MAINTAIN privilege allows statistics manipulation functions.

Database object statistics manipulation functions were introduced
in PostgreSQL 18 and are permitted under the MAINTAIN privilege.
However, the documentation previously did not mention these functions
in the list of allowed operations.

This commit updates the MAINTAIN privilege documentation to
explicitly include statistics manipulation functions, clarifying
what the privilege covers.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Robert Treat <rob@xzilla.net>
Discussion: https://postgr.es/m/7c7e1ad5-fdf9-486f-bc63-40ac99b0461d@oss.nttdata.com