AVRO-4178: [C#] add documentation on --namespace flag of C# AvroGen

[DavidNeukamXXXLutz]

What is the purpose of the change

Add documentation of a not expected behaviour of --namespace flag of C# AvroGen

Wenn tring to fix AVRO-4187, I identified an issue that all the avro tooling stops working when the --namespace flag really does what you would expect from it. I think the flag should be removed entirely in a later version. AvroGen should not have a --namespace flag as the other Avro tooling does not support namespace mappings - so we are depending on a bug in the code generation to be able to deserialize the types again (affecting the contract).

in this PR, I only document this specific behaviour

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable