pgsql: doc: add pg_prewarm example

doc: add pg_prewarm example

Discussion: /messages/by-id/20220618085541.ezxdaljlpo6x7msc@home-desktop

Author: Dong Wook Lee

Backpatch-through: 11

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9d71664c88defe967ccdbf485c0216c0d5a301ee

Modified Files
--------------
doc/src/sgml/pgprewarm.sgml | 13 +++++++++++++
1 file changed, 13 insertions(+)