Document how to configure token-expiration-time

[korgano]

PR 2753 (filebrowser/filebrowser#2753) integrates a variable to configure token-expiration-time.

While the PR has some example configurations, it is not at all clear how to use them, especially if the user is working with a Docker implementation of the app.

At minimum, Docker implementation requires documentation on how to configure this setting to resolve timeout issues on long uploads.