bpo-20923 : [doc] Explain ConfigParser 'valid section name' and .SECTCRE (GH-31413)
diff --git a/Doc/library/configparser.rst b/Doc/library/configparser.rst
index 1ebda53..323dd2a 100644
--- a/Doc/library/configparser.rst
+++ b/Doc/library/configparser.rst
@@ -267,6 +267,9 @@
than the first line of the value. Depending on the parser's mode, blank lines
may be treated as parts of multiline values or ignored.
+By default, a valid section name can be any string that does not contain '\\n' or ']'.
+To change this, see :attr:`ConfigParser.SECTCRE`.
+
Configuration files may include comments, prefixed by specific
characters (``#`` and ``;`` by default [1]_). Comments may appear on
their own on an otherwise empty line, possibly indented. [1]_
