Revise and expand CollectionIncompatibleType documentation.
Revise and expand CollectionIncompatibleType documentation.

- Clarify why the example code is buggy.
- Focus more throughout on why the signature is contains(Object) and not
  contains(E).
- Then talk a ton about that in a new "footnote."
- Attempt to address some comments from CL 107499781. (But I've ignored
  ArrayList vs. LinkedList and List<Object> vs. List<String.)

MOE_MIGRATED_REVID=123033541

google/error-prone,cushon/error-prone,cushon/error-prone,cushon/error-prone,google/error-prone,cushon/error-prone