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
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d4e4cecace91bd91f77da1cf081df2dba1b76a41

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