doc: use `git-secure-tag` for release tags

`git-secure-tag` recursively constructs an SHA-512 digest out of the
git tree, and puts the hash from the tree's root into the tag
annotation. This hash provides better integrity guarantees than the
default SHA-1 merkle tree that git uses.

Fix: #7579
PR-URL: https://github.com/nodejs/node/pull/7603


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>