Improve comment on setting data_checksum GUC.

There was an extra space there, and "fixed" wasn't very descriptive.
doc: Clarify documentation page header customization code

The customization overrode the fast-forward code with its custom Up
link.  So this is no longer really the fast-forward feature, so we might
as well turn that off and override the non-ff template instead, thus
removing one mental indirection.

Fix the wrong column span declaration.

Clarify and update the documentation.