doc: document optional params in napi_get_cb_info

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

Author: mhdawson

doc/api/n-api.md

@@ -4630,14 +4630,18 @@ napi_status napi_get_cb_info(napi_env env,
 * `[in] env`: The environment that the API is invoked under.
 * `[in] cbinfo`: The callback info passed into the callback function.
 * `[in-out] argc`: Specifies the length of the provided `argv` array and
-  receives the actual count of arguments.
+  receives the actual count of arguments. `argc` can
+  optionally be ignored by passing `NULL`.
 * `[out] argv`: Buffer to which the `napi_value` representing the arguments are
   copied. If there are more arguments than the provided count, only the
   requested number of arguments are copied. If there are fewer arguments
   provided than claimed, the rest of `argv` is filled with `napi_value` values
-  that represent `undefined`.
-* `[out] this`: Receives the JavaScript `this` argument for the call.
-* `[out] data`: Receives the data pointer for the callback.
+  that represent `undefined`. `argv` can optionally be ignored by
+  passing `NULL`.
+* `[out] this`: Receives the JavaScript `this` argument for the call. `this`
+  can optionally be ignored by passing `NULL`.
+* `[out] data`: Receives the data pointer for the callback. `data` can
+  optionally be ignored by passing `NULL`.
 
 Returns `napi_ok` if the API succeeded.