doc: clarify async execute callback usage

Clarify that calls to N-API should be avoided in
the 'execute' callback.

PR-URL: https://github.com/nodejs/node/pull/21217
Refs: https://github.com/nodejs/help/issues/1318


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>