doc,tools: formalize, unify, codify default values

PR-URL: https://github.com/nodejs/node/pull/19737
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/string_decoder.md

@@ -48,7 +48,7 @@ added: v0.1.99
 -->
 
 * `encoding` {string} The character encoding the `StringDecoder` will use.
-  Defaults to `'utf8'`.
+  **Default:** `'utf8'`.
 
 Creates a new `StringDecoder` instance.