pgsql: Doc: clean up verify_heapam() documentation.

Doc: clean up verify_heapam() documentation.

I started with the intention of just suppressing a PDF build warning
by removing the example output, but ended up doing more: correcting
factual errors in the function's signature, moving a bunch of
generalized handwaving into the "Using amcheck Effectively" section
which seemed a better place for it, and improving wording and markup
a little bit.

Discussion: https://postgr.es/m/732904.1603728748@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4c49d8fc15eeb1dc69b0ddb2d986a1884a5d7f5f

Modified Files
--------------
doc/src/sgml/amcheck.sgml | 157 +++++++++++++++++++---------------------------
1 file changed, 65 insertions(+), 92 deletions(-)