pgsql: doc: make blooms docs match reality

doc: make blooms docs match reality

Parallel execution changed the way bloom queries are executed, so update
the EXPLAIN output, and restructure the docs to be clearer and more
accurate.

Reported-by: Daniel Westermann

Discussion: /messages/by-id/ZR0P278MB0122119FAE78721A694C30C8D2340@ZR0P278MB0122.CHEP278.PROD.OUTLOOK.COM

Author: Daniel Westermann and me

Backpatch-through: 9.6

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9ad40fe6ef9195c9776b4a54357c4719acdfb0df

Modified Files
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doc/src/sgml/bloom.sgml | 117 +++++++++++++++++++++++++-----------------------
1 file changed, 62 insertions(+), 55 deletions(-)