Default include_dir behaviour

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/13/config-setting.html
Description:

Dear postgres-team,

please consider adding a short line which clarifies what the default
behaviour of the `include_dir` paramter is if it is not set.
I am currently wondering if other .conf files in the default config
directory are included by default as well.

Thank you for your time.
Greetings,
Tobias