add note about os.path.join and drive letters on Windows.

Doc/lib/libposixpath.tex

@@ -123,7 +123,10 @@ Joins one or more path components intelligently.  If any component is
 an absolute path, all previous components are thrown away, and joining
 continues.  The return value is the concatenation of \var{path1}, and
 optionally \var{path2}, etc., with exactly one slash (\code{'/'})
-inserted between components, unless \var{path} is empty.
+inserted between components, unless \var{path} is empty.  Note that on
+Windows, since there is a current directory for each drive,
+{}\function{os.path.join("c:", "foo")} represents a path relative to the
+current directory on drive C:, not \code{c:\\foo}.
 \end{funcdesc}
 
 \begin{funcdesc}{normcase}{path}