n-api: document nextTick timing in callbacks

PR-URL: #33804
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Chengzhong Wu <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Gabriel Schulhof <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>

Author: mafintosh

doc/api/n-api.md

@@ -4751,6 +4751,9 @@ is sufficient and appropriate. Use of the `napi_make_callback` function
 may be required when implementing custom async behavior that does not use
 `napi_create_async_work`.
 
+Any `process.nextTick`s or Promises scheduled on the microtask queue by
+JavaScript during the callback are ran before returning back to C/C++.
+
 ### napi_open_callback_scope
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