SF bug #671447: StringIO doc doesn't say it's sometimes read-only.
diff --git a/Doc/lib/libstringio.tex b/Doc/lib/libstringio.tex
index 9279a3f2..c268690 100644
--- a/Doc/lib/libstringio.tex
+++ b/Doc/lib/libstringio.tex
@@ -59,6 +59,11 @@
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: