Clarify why pathlib doesn't inherit from str

pep-0428.txt

@@ -156,8 +156,16 @@ contrasts with some other Path class proposals which were derived from
 if you want a path to act as a sequence, you have to lookup a dedicated
 attribute (the ``parts`` attribute).
 
-Not behaving like one of the basic builtin types also minimizes the potential
-for confusion if a path is combined by accident with genuine builtin types.
+The key reasoning behind not inheriting from ``str`` is to prevent confusing
+any object that has a string representation -- which is all Python objects --
+with a path object. This avoids situations when people must interface with an
+API that mandates a string representation for a file system path, tempting you
+to use ``str(path)`` to extract the string representation of the path. This
+would be a problem if ``path = None; open(str(path), 'w')``. A `blog post`_ by
+a Python core developer goes into more detail on the reasons behind this
+specific design decision.
+
+.. _blog post: http://www.snarky.ca/why-pathlib-path-doesn-t-inherit-from-str
 
 
 Immutability