[google_sign_in] Add exception info to migration guide

[stuartmorgan-g]

Adds discussion of the new exception flows to the migration guide.

Also makes the API comment discussion of initialize more thorough

Fixes flutter/flutter#171704
Fixes flutter/flutter#171746

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[stuartmorgan-g]

I folded in another minor docs update from another issue that just came in, so that I wouldn't have to do conflict resolution to land them both since they are both simple.

[bparrishMines]

LGTM