pgsql: contrib/bloom documentation improvement

contrib/bloom documentation improvement

This commit documents rounding of "length" parameter and absence of support
for unique indexes and NULLs searching.  Backpatch to 9.6 where contrib/bloom
was introduced.

Discussion: https://postgr.es/m/CAF4Au4wPQQ7EHVSnzcLjsbY3oLSzVk6UemZLD1Sbmwysy3R61g%40mail.gmail.com
Author: Oleg Bartunov with minor editorialization by me
Backpatch-through: 9.6

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a05f7053e842d1463c35713ae79e6795b8751dc2

Modified Files
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doc/src/sgml/bloom.sgml | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)