Also merge Doxygen comments for overloaded constructors in Python.

This was done for plain functions/methods but not constructors, which don't
have "kind=function" attribute.

Just don't check for "kind" at all, the presence of "sym:overloaded" should be
good enough to indicate that it's an overloaded something.

Also add a test for overloaded constructors documentation.

Examples/test-suite/python/doxygen_parsing_runme.py

@@ -17,6 +17,15 @@ commentVerifier.check(doxygen_parsing.SomeStruct.__doc__,
     r"""
     The struct comment
     """)
+commentVerifier.check(doxygen_parsing.SomeAnotherClass.__init__.__doc__,
+    r"""
+        *Overload 1:*
+         First overloaded constructor.
+
+        *Overload 2:*
+         Second overloaded constructor.
+
+        """)
 commentVerifier.check(doxygen_parsing.SomeAnotherClass.classMethod.__doc__,
     r"""
         The class method comment.