SelectionPluginManager class docs should explain how a selection plugin is chosen

Problem/Motivation

SelectionPluginManager has complex logic for determining which plugin it returns from getInstance().

The logic is in getInstance() and getPluginId().

Because these are methods which are inherited from the parent class (and HEAVILY customised), the documentation should go on the class.

Steps to reproduce

Proposed resolution

Remaining tasks

Figure out the logic.
Document it.

User interface changes

API changes

Data model changes

Release notes snippet