doc: write Go 1.16 release notes

[dmitshur]

Tracking bug for writing the Go 1.16 Release Notes.

The latest state on tip can be viewed at https://tip.golang.org/doc/go1.16.

When the Go 1.16 Release Notes are complete, this should be closed, and a similar issue should be made for the next release. Previous issues are #37419, #36878, #17929, #15810, #5929, etc.

[gopherbot]

Change https://golang.org/cl/248198 mentions this issue: doc/go1.16: start draft release notes

[gopherbot]

Change https://golang.org/cl/260337 mentions this issue: doc/go1.16: document removal of GO386=387

[gopherbot]

Change https://golang.org/cl/267718 mentions this issue: doc/go1.16: add release notes for darwin ports

[gopherbot]

Change https://golang.org/cl/272871 mentions this issue: doc/go1.16: fill in Go 1.16 release note TODOs using relnote