Document that PluginSelectorInterface's form methods need matching $form and $form_state parameters

PluginSelectorInterface's form methods need matching $form and $form_state parameters. This is not documented as such, which can lead people to believe passing the global/complete form state is fine.

When #2537732: PluginFormInterface must have access to the complete $form_state (introduce SubFormState for embedded forms) is committed, it will provide an out-of-the-box solution to implementing this interface.