clarify the meaning and purpose of the different widget types

The first page of the admin UI for editing or creating a browser is *really confusing*.

None of the 3 plugin types explain what they do.

I can figure out what 'Display plugin' is about by looking at the options: it's how the entity browser widget it displayed in a form.

'Widget selector plugin' I'm less sure of. Is that the style of UI that allows switching between widgets?

'Selection display plugin' I have NO idea.

I would suggest:

- description text on each of the 3 plugin selection form elements, explaining the purpose of the plugin type.
- switching to radios instead of a select element, which would allow showing descriptions on each radio. The description property of each plugin could be output here.

So for example, the display option done as radios, with an overall description (at the bottom) and a description on each radio:

Display plugin:
(*) iFrame
  The entity browser form is shown inside the host form, using an iFrame.
() Modal
   The entity browser opens in a popup.
() Standalone form
   The user is taken to a new page to use the entity browser, and then is returned to the host form when they have finished.
Determines how the entity browser is shown when used within another form.

(Assuming I have guessed correctly what 'Standalone form' means -- I have *no* idea if that's correct!)