Document if browser addons can be saved in persistent volume

It is not clearly stated in the documentation if browser plugins are part of the “dotfiles” or not.
Browser bookmarks have a separate entry on the page.
https://tails.boum.org/doc/first_steps/persistence/configure/index.en.html

Feature Branch: bitingbird:doc/7593-persistence_limits

Original created by @u on 7593 (Redmine)