doc/go1.16: document runtime changes for Go 1.16

[dmitshur]

As of filing this issue, the Go 1.16 draft release notes contained the following HTML:

Runtime

TODO

On Linux, the runtime now defaults to releasing memory to the operating system promptly (using MADV_DONTNEED), rather than lazily when the operating system is under memory pressure (using MADV_FREE). This means process-level memory statistics like RSS will more accurately reflect the amount of physical memory being used by Go processes. Systems that are currently using GODEBUG=madvdontneed=1 to improve memory monitoring behavior no longer need to set this environment variable.

[...]

runtime

TODO: https://golang.org/cl/37222: make stack traces of endless recursion print only top and bottom 50

TODO: https://golang.org/cl/242258: add 24 byte allocation size class

TODO: https://golang.org/cl/254659: implement GODEBUG=inittrace=1 support

---

Per golang.org/s/release, it's a release blocker for Go 1.16 Beta 1 to have a complete draft of the eventual release notes, and so the TODO needs to be resolved.

A sequence of steps to resolve this issue may be:

1. Come up with a draft release note entry in this issue, then move the issue to NeedsFix state.
2. Send a CL with "doc/go1.16:" prefix and include "For doc: write Go 1.16 release notes #40700. Fixes #{this issue}." in the body.

[gopherbot]

Change https://golang.org/cl/275176 mentions this issue: doc/go1.16: update runtime and compiler sections