doc: use **Default:** more consistently

It seems that **Default:** was unofficially decided on as
the way to specify default values. This commit updates the API
documentation to use this format more consistently.

PR-URL: https://github.com/nodejs/node/pull/37387


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>