Update Histogram<T> docs to note that decimal data will not be exported

[maxs-rose]

Describe the issue or suggestion

As per open-telemetry/opentelemetry-dotnet#5615 when creating a Histogram<decimal> the data will not acutally be exported.

Since the generic type param will accept a decimal it could be best to just add a second remark that when using it the data wont actualy be exported? Of course just removing decimal from the current list of types would also work.

Current Docs: https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.metrics.histogram-1

Happy to make a pull request doing the work however, I am not entirely sure what should be done.

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[dotnet-issue-labeler]

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