pgsql: BRIN: improve documentation on summarization

BRIN: improve documentation on summarization

The existing wording wasn't clear enough and some details weren't
anywhere, such as the fact that autosummarization is off by default.
Improve.

Authors: Roberto Mello, Jaime Casanova, Justin Pryzby, Álvaro Herrera
Discussion: https://postgr.es/m/CAKz==bK_NoJytRyQfX8K-erCW3Ff7--oGYpiB8+ePVS7dRVW_A@mail.gmail.com
Discussion: https://postgr.es/m/20220224193520.GY9008@telsasoft.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7c2b79c2bc22259a2de086af65fc0d8f0cd7a0f2

Modified Files
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doc/src/sgml/brin.sgml             | 76 ++++++++++++++++++++++++++++----------
doc/src/sgml/ref/create_index.sgml |  4 +-
2 files changed, 59 insertions(+), 21 deletions(-)