bug#24842: 24.5; `help-make-xrefs': False link-type determinations

[Drew Adams <drew.adams@oracle.com>]

> What about ZERO WIDTH SPACE?
> 
> `isearchp-movement-unit-alist'\t- units and their movement
> functions\u200B
> `isearchp-on-demand-action-function'\t- on-demand action
> function\u200B
> `isearchp-prompt-for-filter-name'\t- when to ask for filter
> name\u200B

Yes, I thought there was such a character, and it could be
used.  That is a reasonable workaround, I guess, for a
programmer to override inappropriate behavior that can
occur now.

I should have separated #1 and #2 not as alternatives, but
as two approaches that should both, I think, be taken.

Do you think the code as written in help-mode.el is correct?
Do you not think that the cond clauses are just wrong?

Note that only a couple of the cond clauses have the weird
form of putting part of the test for type in the result,
and not in the test part.

The other clauses have, as one might expect, all of the
testing in the condition part, and only the `help-xref-button'
call in the result part.

(That doesn't prove that the same approach should be taken
for all of them.  But it does suggest a possible miscoding.)

I think it would be good to take a close look at this.

And also to document the workaround, letting people know
of a _recommended_ way to override the default behavior.