pgsql: Doc: document cases where queryid is stable

Doc: document cases where queryid is stable

The documents were clear that queryid should not be assumed to be stable
between major versions but said nothing about minor versions and left
the reader to guess if that was implied by the mention of the
instability of queryid between major versions.

Here we give minor versions an explicit mention to indicate queryid can
generally be assumed stable between minor versions.

Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CAApHDvpYGE6h0cD9UO-eHySPynPj1L3J%3DHxT%2BA7Ud8_Yo6AuzA%40mail.gmail.com
Backpatch-through: 12

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/74a587a009d134a92dea12f1971cca9165ae0a26

Modified Files
--------------
doc/src/sgml/pgstatstatements.sgml | 24 +++++++++++++++---------
1 file changed, 15 insertions(+), 9 deletions(-)