Session Timeout
Umbraco is set up to automatically log a user out if they have been inactive for over 20 minutes. Don't worry if you are logged out of Umbraco. Log back in using your credentials and continue editing.
You are currently looking at documentation for Umbraco 8 and older versions.
An automated guess is that docs.umbraco.com/umbraco-cms/tutorials/editors-manual/tips-and-tricks/session-timeout/ could be the link to the new documentation for Umbraco 9 and newer versions.
Umbraco is set up to automatically log a user out if they have been inactive for over 20 minutes. Don't worry if you are logged out of Umbraco. Log back in using your credentials and continue editing.