PR Summary

This PR prevents users from stumbling into #7122, #8779, #15501.
It does not technically fix them, FixedFormatter has a one-off bug with locators other than FixedLocator. But setting fixed labels at unknown positions is anyway not a good idea. So even if the one-off bug was fixed FixedFormatter should only used with FixedLocator.

• This PR mostly documents that FixedFormatter should not be used without a FixedLocator (along the hierarchy of tick label setter methods).
• Additionally it warns on the lowest level set_major/minor_formatter() if a FixedFormatter is set without a FixedLocator. This may be debatable.
  One could set the formatter first and the locator afterwards, which would technically yield the correct result. However, I feel that setting the position first and the labels second is the natural way, and is actually done throughout the library.
  No alternative: Since Formatters don't know about Locators FixedFormatter itself cannot check at runtime and we have to resort to this somewhat clumsy approach.
  And I strongly want some feedback on this potential problem from running code, because nobody reads the docs 😄.
• One could consider captureing and re-emitting warnings on the higher levels with more appropriate messages, but I felt this was too much fuss.