SF bug #671447: StringIO doc doesn't say it's sometimes read-only.

Doc/lib/libstringio.tex

@@ -59,6 +59,11 @@ Unlike the memory files implemented by the \refmodule{StringIO}
 module, those provided by this module are not able to accept Unicode
 strings that cannot be encoded as plain \ASCII{} strings.
 
+Another difference from the \refmodule{StringIO} module is that calling
+\function{StringIO()} with a string parameter creates a read-only object.
+Unlike an object created without a string parameter, it does not have
+write methods.
+
 The following data objects are provided as well: