Java: Deprecate and remove Element::getDoc

[Marcono1234]

Element::getDoc only performs an implicit cast of this:

codeql/java/ql/src/semmle/code/java/Element.qll

Lines 42 to 43 in f9973d1

	/** Cast this element to a `Documentable`. */
	Documentable getDoc() { result = this }

• This is probably redundant because you can instead write a cast element.(Document) ...
• It is irritating because it suggests that every Element can be 'converted' to Document, which is not the case
• It causes irritating error messages when used on types which cannot be cast to Document:
  

Therefore it might be good to deprecate and remove the predicate.

Edit: Package is not a good example for an Element which does not extend Document since it should support documentation, see #5288. Though there are other Element subclasses such as Import or Modifier which cannot be documented in Java.

Relates to #3705

[aschackmull]

I agree that it would be nice to clean this up a bit, as getDoc indeed shouldn't be available on Package. However, we would like to have it available on both Member, Class, and Interface, and their nearest common ancestor is currently Element so that's why getDoc is defined there. Cleaning this up in a nice way requires a little bit thought and some work and is fairly low priority, so we may not immediately fix this.

[github-actions]

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