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

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