doc: more on help2man and "Errors with distclean".

doc: more on help2man and "Errors with distclean".

This fixes (the rest of) https://bugs.gnu.org/67891.

* doc/automake.texi (Errors with distclean): make a subsection of
"Checking the Distribution". Contrast help2man usage examples of
Autoconf (includes man pages in distribution) and
Automake (no man pages, distributes the help2man script instead).
(Checking the Distribution): convert subheadings to subsections.
* doc/local.mk: also mention autoconf vs. automake approaches here,
and point to the manual.
(checklinkx): en passant, remove unused checklinkx exclude.