issue1172: Documentation of "done" attribute in cgi module.

Doc/library/cgi.rst

@@ -147,6 +147,11 @@ attribute::
            if not line: break
            linecount = linecount + 1
 
+If an error is encountered when obtaining the contents of an uploaded file
+(for example, when the user interrupts the form submission by clicking on
+a Back or Cancel button) the :attr:`done` attribute of the object for the
+field will be set to the value -1.
+
 The file upload draft standard entertains the possibility of uploading multiple
 files from one field (using a recursive :mimetype:`multipart/\*` encoding).
 When this occurs, the item will be a dictionary-like :class:`FieldStorage` item.