pgsql: doc: Add note about possible performance overhead by enabling tr

doc: Add note about possible performance overhead by enabling track_planning.

Enabling pg_stat_statements.track_plaanning may incur a noticeable
performance penalty, especially when a fewer kinds of queries are executed
on many concurrent connections. This commit documents this note.

Back-patch to v13 where pg_stat_statements.track_plaanning was added.

Suggested-by: Pavel Stehule
Author: Fujii Masao
Reviewed-by: Pavel Stehule
Discussion: https://postgr.es/m/CAFj8pRC9Jxa8r5i0TNBWLb8mzuaYzEoLq3QOvip0jVpHPOLbVA@mail.gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/da6b6ff95bcaadc109ab248471527a2511e853d5

Modified Files
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doc/src/sgml/pgstatstatements.sgml | 3 +++
1 file changed, 3 insertions(+)