Indicate that os.environ may not be a real dictionary.

Doc/lib/libposix.tex

@@ -35,8 +35,8 @@ below.
 Module \module{posix} defines the following data items:
 
 \begin{datadesc}{environ}
-A dictionary representing the string environment at the time
-the interpreter was started.
+A dictionary or dictionary look-alike representing the string
+environment at the time the interpreter was started.
 For example,
 \code{posix.environ['HOME']}
 is the pathname of your home directory, equivalent to