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: #20390
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Gabriel Schulhof <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>