doc: bring releases.md up to date
This commit brings the release procedure documentation up to date. I tried to consolidate the material in the existing releases.md
with https://github.com/nodejs/node/issues/3254 and https://github.com/nodejs/node/pull/4384#issuecomment-166877340.
Refs: https://github.com/nodejs/node/issues/3254 R= @nodejs/release