docs: add InvalidChars filter explanation

Author: kenjis

user_guide_src/source/incoming/filters.rst

@@ -204,6 +204,14 @@ The filters bundled with CodeIgniter4 are: ``Honeypot``, ``CSRF``, ``InvalidChar
 
 .. note:: The filters are executed in the order defined in the config file. However, if enabled, ``DebugToolbar`` is always executed last because it should be able to capture everything that happens in the other filters.
 
+InvalidChars
+=============
+
+This filter checks if user input data (``$_GET``, ``$_POST``, ``$_COOKIE``, ``php://input``) do not contain the following characters:
+
+- invalid UTF-8 characters
+- control characters except line break and tab code
+
 SecureHeaders
 =============