SF bug #1180392: StringIO's docs should mention overwriting of initial value

* Added a note that the initial file position is zero even if the object
  is freshly initialized.

Doc/lib/libstringio.tex

@@ -14,6 +14,7 @@ files}).  See the description of file objects for operations (section
 When a \class{StringIO} object is created, it can be initialized
 to an existing string by passing the string to the constructor.
 If no string is given, the \class{StringIO} will start empty.
+In both cases, the initial file position starts at zero.
 
 The \class{StringIO} object can accept either Unicode or 8-bit
 strings, but mixing the two may take some care.  If both are used,