Document __bytes__ in PurePath docstring

Author: barneygale

Lib/pathlib.py

@@ -674,6 +674,11 @@ class PurePath(_BasePurePath):
     instantiating a PurePath will return either a PurePosixPath or a
     PureWindowsPath object.  You can also instantiate either of these classes
     directly, regardless of your system.
+
+    On Posix, calling ``bytes()`` on a path gives the raw filesystem path as
+    a bytes object, as encoded by ``os.fsencode()``. On Windows, the unicode
+    form is the canonical representation of fileysstem paths, and so calling
+    ``bytes()`` on a path is not recommended.
     """
     __slots__ = ()