Add docs about TOKEN_USER_CLASS

Author: Andrew-Chen-Wang

docs/settings.rst

@@ -37,6 +37,7 @@ Some of Simple JWT's behavior can be customized through settings variables in
 
       'AUTH_TOKEN_CLASSES': ('rest_framework_simplejwt.tokens.AccessToken',),
       'TOKEN_TYPE_CLAIM': 'token_type',
+      'TOKEN_USER_CLASS': 'rest_framework_simplejwt.models.TokenUser',
 
       'JTI_CLAIM': 'jti',
 
@@ -235,6 +236,14 @@ identifier is used to identify revoked tokens in the blacklist app.  It may be
 necessary in some cases to use another claim besides the default "jti" claim to
 store such a value.
 
+``TOKEN_USER_CLASS``
+--------------------
+
+A stateless user object which is backed by a validated token. Used only for
+the experimental JWTTokenUserAuthentication authentication backend. The value
+is a dotted path to your subclass of ``rest_framework_simplejwt.models.TokenUser``,
+which also is the default.
+
 ``SLIDING_TOKEN_LIFETIME``
 --------------------------