[ffi/doc] Document the type requirements of native functions

Closes: https://github.com/dart-lang/sdk/issues/53623
CoreLibraryReviewExempt: VM and WASM API only, only updates doc.
Change-Id: Id7378da13aaac3546c9c227ed568ff346e33dddf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/328260
Reviewed-by: Lasse Nielsen <lrn@google.com>
Auto-Submit: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

sdk/lib/ffi/ffi.dart

@@ -1134,7 +1134,7 @@ abstract final class NativeApi {
 /// NOTE: This experimental feature is replaced by [Native].
 @Since('2.14')
 @Deprecated('Use Native instead.')
-final class FfiNative<T> {
+final class FfiNative<T extends Function> {
   final String nativeName;
 
   /// Whether the function is a leaf function.
@@ -1173,8 +1173,9 @@ final class FfiNative<T> {
 /// are resolved against an asset, and to, optionally, provide overrides
 /// of the default symbol and asset IDs.
 ///
-/// The type argument to the [Native] annotation must be a function type
+/// The type argument [T] to the [Native] annotation must be a function type
 /// representing the native function's parameter and return types.
+/// The parameter and return types must be subtypes of [NativeType].
 ///
 /// Example:
 ///

sdk/lib/ffi/native_type.dart

@@ -140,6 +140,8 @@ abstract final class Handle extends NativeType {}
 
 /// Represents a function type in C.
 ///
+/// The return type and argument types in [T] must be a subtype of [NativeType].
+///
 /// [NativeFunction] is not constructible in the Dart code and serves purely as
 /// marker in type signatures.
 @unsized