Provide more control over automatic generation of docstrings (#486)

Added the docstring_options class, which gives global control over the generation of docstrings and function signatures.

tests/test_docstring_options.py

@@ -0,0 +1,32 @@
+from pybind11_tests import ConstructorStats
+
+def test_docstring_options(capture):
+    from pybind11_tests import (test_function1, test_function2, test_function3,
+                                test_function4, test_function5, test_function6,
+                                test_function7, DocstringTestFoo)
+
+    # options.disable_function_signatures()
+    assert not test_function1.__doc__
+
+    assert test_function2.__doc__ == "A custom docstring"
+
+    # options.enable_function_signatures()
+    assert test_function3.__doc__ .startswith("test_function3(a: int, b: int) -> None")
+
+    assert test_function4.__doc__ .startswith("test_function4(a: int, b: int) -> None")
+    assert test_function4.__doc__ .endswith("A custom docstring\n")
+
+    # options.disable_function_signatures()
+    # options.disable_user_defined_docstrings()
+    assert not test_function5.__doc__
+
+    # nested options.enable_user_defined_docstrings()
+    assert test_function6.__doc__ == "A custom docstring"
+
+    # RAII destructor
+    assert test_function7.__doc__ .startswith("test_function7(a: int, b: int) -> None")
+    assert test_function7.__doc__ .endswith("A custom docstring\n")
+
+    # Suppression of user-defined docstrings for non-function objects
+    assert not DocstringTestFoo.__doc__
+    assert not DocstringTestFoo.value_prop.__doc__