doc: add pending-deprecation to deprecations list

This adds a mention of `--pending-deprecation` flag effect
in the Deprecated APIs document, and explicitly labels
deprecations that support it.

Refs: https://github.com/nodejs/node/issues/18417
PR-URL: https://github.com/nodejs/node/pull/18433


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>