Very minor tweaks (by-product of reviewing PR #3719).

Author: rwgk

.github/CONTRIBUTING.md

@@ -93,7 +93,7 @@ cmake --build build -j4
 
 Tips:
 
-* You can use `virtualenv` (faster, from PyPI) instead of `venv`
+* You can use `virtualenv` (faster, from PyPI) instead of `venv`.
 * You can select any name for your environment folder; if it contains "env" it
   will be ignored by git.
 * If you don't have CMake 3.14+, just add "cmake" to the pip install command.

include/pybind11/pybind11.h

@@ -1213,7 +1213,7 @@ class module_ : public object {
         PyModule_AddObject(ptr(), name, obj.inc_ref().ptr() /* steals a reference */);
     }
 
-    using module_def = PyModuleDef;
+    using module_def = PyModuleDef; // TODO: Can this be removed (it was needed only for Python 2)?
 
     /** \rst
         Create a new top-level module that can be used as the main module of a C extension.
@@ -1241,7 +1241,7 @@ class module_ : public object {
             pybind11_fail("Internal error in module_::create_extension_module()");
         }
         // TODO: Should be reinterpret_steal for Python 3, but Python also steals it again when
-        // returned from PyInit_...
+        //       returned from PyInit_...
         //       For Python 2, reinterpret_borrow was correct.
         return reinterpret_borrow<module_>(m);
     }