Clarify Elements.overrides

While strongly hinted, it's not strictly specified in JLS that interface methods whose signatures are override-equivalent to signatures of public, non-final methods of java.lang.Object do, in fact, override the latter.

For example, java.lang.CharSequence#toString for practical purposes can be seen to override java.lang.Object#toString.

Elements.overrides should clarify that in its specification, as it might not be obvious or clear from the references to JLS sections it makes.