[native_assets_cli] [doc] Document multiple invocation behavior

[dcharkes]

Document the behavior of multiple invocations of hooks.

Closes: #1739

We could keep buildAssetTypes as a list of strings for now. We might want to make link hooks work with multiple asset types later as a non-breaking change.

Both Dart and Flutter only pass [CodeAsset.type], so they are already satisfying that every asset type has its own invocation.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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Unrelated files missing license headers

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 87.412% (+0.01%) from 87.4%
when pulling da7f8eb on multiple-invocations-documentation
into f179ec2 on main.

[mkustermann]

Maybe we can drop this part and only have the one below?