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 <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>

Author: vsemozhetbyt