[doc] adding way to examine the plan type of prepared statements

Hi,


Currently, EXPLAIN is the only way to know whether the plan is generic 
or custom according to the manual of PREPARE.

   https://www.postgresql.org/docs/devel/sql-prepare.html

After commit d05b172, we can also use pg_prepared_statements view to 
examine the plan types.

How about adding this explanation like the attached patch?


Regards,

--
Atsushi Torikoshi