Document Dovecot auth

DOCUMENTATION.md

@@ -746,6 +746,9 @@ Available backends:
 `ldap`
 : Use a LDAP or AD server to authenticate users.
 
+`dovecot`
+: Use a local Dovecot server to authenticate users.
+
 Default: `none`
 
 ##### htpasswd_filename
@@ -858,6 +861,12 @@ The path to the CA file in pem format which is used to certificate the server ce
 
 Default:
 
+##### dovecot_socket
+
+The path to the Dovecot client authentication socket (eg. /run/dovecot/auth-client on Fedora). Radicale must have read / write access to the socket.
+
+Default:
+
 ##### lc_username
 
 Сonvert username to lowercase, must be true for case-insensitive auth