chore(doclint): support classes inheritance (#935)

This patch:
- gives meaningful names to doclint tests
- supports classes inheritance in documentation linter. When class A
  extends class B, all methods of class B are added to documentation of
  class A.

This is a prerequisite for Object Handles: ElementHandle will be
extending ObjectHandle.

References #382

utils/doclint/check_public_api/test/diff-methods/result.txt

@@ -0,0 +1,3 @@
+[MarkDown] Non-existing method found: Foo.proceed()
+[MarkDown] Method not found: Foo.stop()
+[MarkDown] Property not found: Foo.zzz