doc: update napi_async_init documentation

Fixes: nodejs#33153

Change documentation to make async_resource required
as opposed to optional in napi-async_init.

Changes over time mean this parameter is required for
proper operation of async hooks (which are still experimental).
This changes the documentation to document what
callers should do. We are doing this only in the doc
in order to avoid a breaking change in N-API. We could
create a new version of the method for which the
parametrer is enforced as mandatory but we should only
do that once async hooks is no longer experimental. In
that case we could deprecate (but not remove this version
of the method).

Signed-off-by: Michael Dawson <[email protected]>

doc/api/n-api.md

@@ -4672,7 +4672,7 @@ napi_status napi_async_init(napi_env env,
 ```
 
 * `[in] env`: The environment that the API is invoked under.
-* `[in] async_resource`: An optional object associated with the async work
+* `[in] async_resource`: Object associated with the async work
   that will be passed to possible `async_hooks` [`init` hooks][].
 * `[in] async_resource_name`: Identifier for the kind of resource
   that is being provided for diagnostic information exposed by the
@@ -4681,6 +4681,12 @@ napi_status napi_async_init(napi_env env,
 
 Returns `napi_ok` if the API succeeded.
 
+In order to retain ABI compatibility with previous versions,
+passing `NULL` for `async_resource` will not result in an error, however,
+this will result incorrect operation of async hooks for the
+napi_async_context created. Potential issues include include
+loss of async context when using the AsyncLocalStorage API.
+
 ### napi_async_destroy
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