Doc: subdivide System Information Functions and Operators.

Doc: subdivide System Information Functions and Operators.

Provide <sect2> subdivisions in 9.26 System Information Functions and
Operators.  This is useful because it adds a mini-TOC at the top of
the page to aid jumping to portions of what's become quite a long
section.  Also, now that several of the subsections contain multiple
tables, it's hard to see the overall structure without headings.

Discussion: https://postgr.es/m/4026789.1670426602@sss.pgh.pa.us