docs: oauth improvements

Author: ivictbor

adminforth/documentation/docs/tutorial/05-Plugins/11-oauth.md

@@ -1,6 +1,7 @@
 # OAuth Authentication
 
-The OAuth plugin enables OAuth2-based authentication in AdminForth, allowing users to sign in using their Google, GitHub, Facebook or other OAuth2 provider accounts.
+The OAuth plugin enables OAuth2-based authentication in AdminForth, allowing users to sign in using their Google, GitHub, Facebook or other OAuth2 provider accounts. 
+Optionaly, you can also enable open signup for new users and assign some default attributes, for example low-permission role for users who will sign up using OAuth.
 
 ## Installation