Clarify when `icecandidatepairremove` is fired.

[sam-vi]

Fixes: #196.

Enumerate examples of situations in which icecandidatepairremove is fired, including ICE restart.

This makes it clear that the event is fired and [[CandidatePairs]] is cleared in the event of an ICE restart.

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[jan-ivar]

@sam-vi still seeing some validation errors. Do you need help on this one?

[sam-vi]

@sam-vi still seeing some validation errors. Do you need help on this one?

I have filed #217 to address the validation errors, which are unrelated to this PR itself.