APPS/{x509,req}: Fix description and diagnostics of -key, -in, etc. options

[DDvO]

Over the years, the x509 and req app options have become a mess
and their documentation and help output is partly outdated, incomplete, or even wrong.
I recently came across this issue again when extending the tests in #16342.

This PR improves the description of the -key, -in, and related options
and adds some warnings for useless or problematic option combinations.

Note that this does not change the semantics of the options.
It would be good to make them more consistent or at least flag some combinations as error,
but we likely cannot do this before the 3.0 release.

• documentation is added or updated

[t8m]

OTC: not blocking 3.0.0 release. OTC will decide later if this is acceptable for 3.0 branch.

[openssl-machine]

This pull request is ready to merge

[DDvO]

Can one meanwhile merge to master again as usual?

[paulidale]

Yes, you can merge to master. We're still a while from determining futures though but a change like this seems reasonable to include now.

[DDvO]

Merged - thanks @paulidale

[DDvO]

Yes, you can merge to master. We're still a while from determining futures though but a change like this seems reasonable to include now.

Is it already clear if/how to potentially (back-)port this to 3.0?
Or are documentation improvements not considered important enough for that?
Re-opening this for now, just as a reminder until this is clarified.

[t8m]

It is not a pure documentation fix. It would be probably better to open a new PR against 3.0 branch if you want OTC to consider this.

[paulidale]

I've flagged this for OTC discussion with respects to merging all of this or just the documentation changes to the 3.0 branch.