explain fediverse terminology in UI

Problem/Motivation

Form elements that let you pick a fediverse thingy are really confusing, e.g.

> ActivityPub activity

> ActivityPub object

and even the Type Plugin.

It would help enormously if the values in these form elements had more description to say what these all mean.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes