pgsql: doc: Note that plan advice does not affect cached plans

doc: Note that plan advice does not affect cached plans

Plan advice is consulted only at plan time. When a cached plan is
re-executed, no re-planning occurs, so updated advice has no effect.
DISCARD PLANS can be used to discard cached plans, but only affects
the sessions in which it is executed.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Suggested-by: Jakub Wartak <jakub(dot)wartak(at)enterprisedb(dot)com>
Discussion: https://postgr.es/m/20260827171830.68.noahmisch@microsoft.com
Co-authored-by: Dongpo Liu <poe(dot)liu(at)pm(dot)me>
Co-authored-by: Robert Haas <rhaas(at)postgresql(dot)org>
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a69ac3559823a81fb9b16aa77db450617ff80f14

Modified Files
--------------
doc/src/sgml/pgplanadvice.sgml | 12 ++++++++++++
doc/src/sgml/pgstashadvice.sgml | 8 ++++++++
2 files changed, 20 insertions(+)