pgsql: doc: rewrite random_page_cost description

doc:  rewrite random_page_cost description

This removes some of the specifics of how the default was set, and adds
a mention of latency as a reason the value is lower than the storage
hardware might suggest.  It still mentions caching.

Discussion: https://postgr.es/m/CAKAnmmK_nSPYr53LobUwQD59a-8U9GEC3XGJ43oaTYJq5nAOkw@mail.gmail.com

Backpatch-through: 13

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/6b4b8d5f359b24ccfa16093ed7051d5034fd1344

Modified Files
--------------
doc/src/sgml/config.sgml | 36 ++++++++++++++++++------------------
1 file changed, 18 insertions(+), 18 deletions(-)