doc: indicate n-api out params that may be NULL

This is useful information to have for applications that don't need to read the other properties. The implementation checks for `nullptr`, see: https://github.com/nodejs/node/blob/master/src/js_native_api_v8.cc#L2879

PR-URL: #40371
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/n-api.md

@@ -2821,6 +2821,8 @@ Returns `napi_ok` if the API succeeded.
 
 This API returns various properties of a typed array.
 
+Any of the out parameters may be `NULL` if that property is unneeded.
+
 *Warning*: Use caution while using this API since the underlying data buffer
 is managed by the VM.
 
@@ -2851,6 +2853,8 @@ napi_status napi_get_dataview_info(napi_env env,
 
 Returns `napi_ok` if the API succeeded.
 
+Any of the out parameters may be `NULL` if that property is unneeded.
+
 This API returns various properties of a `DataView`.
 
 #### napi_get_date_value