Add documentation to embedder locale callback (flutter#25437)

The FlutterComputePlatformResolvedLocaleCallback does not pass the
standard user_data baton. This was an unintentional omission in the
original patch.

Add documentation to that effect and link to the bug report:
flutter#79826

Author: cbracken

shell/platform/embedder/embedder.h

@@ -1121,6 +1121,14 @@ typedef struct {
   const char* variant_code;
 } FlutterLocale;
 
+/// Callback that returns the system locale.
+///
+/// Embedders that implement this callback should return the `FlutterLocale`
+/// from the `supported_locales` list that most closely matches the
+/// user/device's preferred locale.
+///
+/// This callback does not currently provide the user_data baton.
+/// https://github.com/flutter/flutter/issues/79826
 typedef const FlutterLocale* (*FlutterComputePlatformResolvedLocaleCallback)(
     const FlutterLocale** /* supported_locales*/,
     size_t /* Number of locales*/);