pgsql: doc: Add missing index entries and fix title formatting in pg_bu

doc: Add missing index entries and fix title formatting in pg_buffercache docs.

This commit adds missing index entries for the functions pg_buffercache_numa()
and pg_buffercache_usage_counts() in the pg_buffercache documentation.

It also makes the function titles consistent by adding parentheses after
function names where they were previously missing.

Author: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/7d19af4b-7da3-4862-9f52-ff958960bd8d@oss.nttdata.com
Backpatch-through: 18

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/086b9a33aafeeeaa0af0c806410ea6228f2f63f4

Modified Files
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doc/src/sgml/pgbuffercache.sgml | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)