#1934: fix os.path.isabs docs.

Doc/library/os.path.rst

@@ -144,7 +144,9 @@ write files see :func:`open`, and for accessing the filesystem see the
 
 .. function:: isabs(path)
 
-   Return ``True`` if *path* is an absolute pathname (begins with a slash).
+   Return ``True`` if *path* is an absolute pathname.  On Unix, that means it
+   begins with a slash, on Windows that it begins with a backslash after chopping
+   off a potential drive letter.
 
 
 .. function:: isfile(path)