event: improve package documentation
event: improve package documentation

Fixes golang/go#47154

Change-Id: I178ae897af03a9bf284e3e8110d3b1440e5b978e
Reviewed-on: https://go-review.googlesource.com/c/exp/+/334489
Trust: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
Run-TryBot: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

golang/exp,golang/exp,golang/exp