Document somewhat the session_bag tag

Problem/Motivation

We implemented a JS messenger service in custom code much like the core messenger service but it didn't work and it was not at all evident why. session.flash_bag is the only service using the FlashBag class and it gets special treatment by passing it as an argument to the session service.

Steps to reproduce

Proposed resolution

Add a note to session.flash_bag to use the tag.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet