Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

Attached is v45 of the patchset. I've done some additional code cleanup
and changes. The most significant change, however, is the docs. I've
separated the docs into its own patch for ease of review.

The docs patch here was edited and co-authored by Samay Sharma.
I'm not sure if the order of pg_stat_io in the docs is correct.

The significant changes are removal of all "correspondence" or
"equivalence"-related sections (those explaining how other IO stats were
the same or different from pg_stat_io columns).

I've tried to remove references to "strategies" and "Buffer Access
Strategy" as much as possible.

I've moved the advice and interpretation section to the bottom --
outside of the table of definitions. Since this page is primarily a
reference page, I agree with Samay that incorporating interpretation
into the column definitions adds clutter and confusion.

I think the best course would be to have an "Interpreting Statistics"
section.

I suggest a structure like the following for this section:
- Statistics Collection Configuration
- Viewing Statistics
- Statistics Views Reference
- Statistics Functions Reference
- Interpreting Statistics

As an aside, this section of the docs has some other structural issues
as well.

For example, I'm not sure it makes sense to have the dynamic statistics
views as sub-sections under 28.2, which is titled "The Cumulative
Statistics System."

In fact the docs say this under Section 28.2
https://www.postgresql.org/docs/current/monitoring-stats.html

"PostgreSQL also supports reporting dynamic information about exactly
what is going on in the system right now, such as the exact command
currently being executed by other server processes, and which other
connections exist in the system. This facility is independent of the
cumulative statistics system."

So, it is a bit weird that they are defined under the section titled
"The Cumulative Statistics System".

In this version of the patchset, I have not attempted a new structure
but instead moved the advice/interpretation for pg_stat_io to below the
table containing the column definitions.

- Melanie