Add example for the more common and confusing IL2111 cases

[jtschuster]

Summary

While going through warnings I saw that this page only describes the trivial case but misses the more confusing case of annotations triggering the warning. I think it would be nice to add a bit more info about it.

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Internal previews

📄 File	🔗 Preview link
docs/core/deploying/trimming/trim-warnings/il2111.md	IL2111: Method with parameters or return value with 'DynamicallyAccessedMembersAttribute' is accessed via reflection. Trimmer cannot guarantee availability of the requirements of the method

[BillWagner]

ping @adegeo for review