doc: add missing backticks in n-api.md

Codify types, variable names, and code fragments
checking patterns I've managed to think of.

Some nits were also fixed in passing
(add missing periods, remove extra line breaks etc).

PR-URL: https://github.com/nodejs/node/pull/20390


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>