[Bug #1441864] Clarify description of 'data' argument

Doc/lib/liburllib2.tex

@@ -18,11 +18,13 @@ The \module{urllib2} module defines the following functions:
 Open the URL \var{url}, which can be either a string or a \class{Request}
 object.
 
-\var{data} should be a string, which specifies additional data to
-send to the server. In HTTP requests, which are the only ones that
-support \var{data}, it should be a buffer in the format of
-\mimetype{application/x-www-form-urlencoded}, for example one returned
-from \function{urllib.urlencode()}.
+\var{data} should be a string, which specifies additional data to send
+to the server. Currently HTTP requests are the only ones that use
+\var{data}.  For HTTP, the request will be a POST instead of a GET
+when the \var{data} parameter is provided.  \var{data} should be a
+buffer in the standard \mimetype{application/x-www-form-urlencoded} format.
+The \function{urllib.urlencode()} function takes a mapping or 
+sequence of 2-tuples and returns a string in this format.
 
 This function returns a file-like object with two additional methods: