doc: Fix link text for data checksums

doc: Fix link text for data checksums

Commit 67846550dc6d removed the xreflabels for initdb options, which
turned the sentence "The second field contains the page checksum if
data checksums are enabled" into "The second field contains the page
checksum if -k are enabled", as well "Only has effect if data checksums
are enabled" into "Only has effect if -k are enabled".

Fix by setting an explicit link text, and while there also change the
link to point to the data checksum page which has more information
than just the initdb option.

The original report was for one instance, further inspection turned
up quite a few more cases.  Also redirect the link in the amcheck
docs which albeit was reading right, but will be more helpful if
linking to the main page on data checksums.  Backpatch to v18 where
the xreflabels were removed.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: y.saburov@gmail.com
Reviewed-by: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://postgr.es/m/178350739237.73862.4549076173872335741@wrigleys.postgresql.org
Backpatch-through: 18