Fully document serialEvent() support for all Arduino boards

[per1234]

serialEvent() support across the official Arduino boards is quite patchy. Previously, the support accross these boards was not completely (or correctly in the case of the Due) documented.

Screenshot of rendered table from local build of reference:

---

Fixes https://github.com/arduino/reference-en/issues/819

[kengdahl]

@per1234 I can't review this because I don't have the knowledge. Do you know who we could assign this to?

[karlsoderby]

This article was updated through these PRs, first https://github.com/arduino/reference-en/pull/952 and later https://github.com/arduino/reference-en/pull/951.

Hardware tests were on several board to confirm that they support / do not support serialEvent().