doc: add links between cork() and uncork()

Writable.cork() and Writable.uncork() are meant to be documented
together, but we maintain a lexicographic order. This commit
introduces some links between the two.

Fixes: https://github.com/nodejs/node/issues/7340
PR-URL: https://github.com/nodejs/node/pull/11222


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>