pgsql: doc: PG 12 relnotes - add item about amcheck index root check

doc: PG 12 relnotes - add item about amcheck index root check

Reported-by: Peter Geoghegan

Discussion: /messages/by-id/LGbT2ncB7tiDsndK0eXHTKmogLjJ5rO52HqXigP8bCA@mail.gmail.com

Backpatch-through: 12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ba2347a6d985afc19d4c8e8e84b16a28ed4df137

Modified Files
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doc/src/sgml/release-12.sgml | 12 ++++++++++++
1 file changed, 12 insertions(+)