Fix extra docstring newlines under options.disable_function_signatures()

When using pybind::options to disable function signatures, user-defined
docstrings only get appended if they exist, but newlines were getting
appended unconditionally, so the docstring could end up with blank lines
(depending on which overloads, in particular, provided docstrings).

This commit suppresses the empty lines by only adding newlines for
overloads when needed.

tests/test_docstring_options.py

@@ -3,13 +3,23 @@
 def test_docstring_options():
     from pybind11_tests import (test_function1, test_function2, test_function3,
                                 test_function4, test_function5, test_function6,
-                                test_function7, DocstringTestFoo)
+                                test_function7, DocstringTestFoo,
+                                test_overloaded1, test_overloaded2, test_overloaded3)
 
     # options.disable_function_signatures()
     assert not test_function1.__doc__
 
     assert test_function2.__doc__ == "A custom docstring"
 
+    # docstring specified on just the first overload definition:
+    assert test_overloaded1.__doc__ == "Overload docstring"
+
+    # docstring on both overloads:
+    assert test_overloaded2.__doc__ == "overload docstring 1\noverload docstring 2"
+
+    # docstring on only second overload:
+    assert test_overloaded3.__doc__ == "Overload docstr"
+
     # options.enable_function_signatures()
     assert test_function3.__doc__ .startswith("test_function3(a: int, b: int) -> None")