[ffigen] Improve docs of config API

[liamappelbe]

Fixes #2769

[github-actions]

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pkgs/objective_c/lib/src/ns_input_stream.dart

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[coveralls]

coverage: 91.965% (+0.04%) from 91.924%
when pulling 398f98b on ffigen_config_docs
into 00991b9 on main.