Improve documentation for current_setting function

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/11/functions-admin.html
Description:

Hi.

Doc is not clear about what is returned if current_setting(
'app.not_existent', true ) is called.

Does it return empty string '', because return type is 'text' or it return
NULL, because 'app.not_existent' does not exist

Thank you.