[cs] Mention inlineNested in perf guidance
[cs] Mention inlineNested in perf guidance

A number of recent support requests (for example, [issue
\#1083](https://github.com/microsoft/bond/issues/1083)) have been
resolved by using `inlineNested: false` when creating
`Serializer`/`Deserializer` instances.

Add details about `inlineNested` to the C# performance guidance.
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