[PR #2438/02282b8d backport][stable-2.17] Update interpreter_discovery for changes made in ansible-core 2.17

[patchback]

This is a backport of PR #2438 as merged into devel (02282b8).

I've updated ansible-core to 2.18, and suddenly got a warning regarding interpreter discovery, which pointed me to this page.

I've read the page and stayed confused why it suddenly happened.

Then I've read the source, and mainly this PR merged in 2.17 changed a lot regarding interpreter discovery.

• There is no "target OS platform, distribution, and version" list anymore (well, it's empty), so you're always using the fallback list if you don't manually specify an interpreter, and so you'll always get a warning.
• Therefore this exception is always raised, and the check for legacy never happens, making auto_legacy effectively an alias for auto.
• Not really important anymore, since the code is never reached, but auto_legacy now uses python3, so seems the legacy option isn't really legacy anymore.

So I've updated the documentation page to reflect these changes.