Warn when using `returns` XML doc comment for void methods

Violation example:

https://github.com/dotnet/roslyn/blob/ab4cbf6de14bebf0b90771aaa1de52d780984e58/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerExecutor.cs#L1232-L1236

(which in itself is appreciated to get fixed)

I don't think `` XML element make sense for a method that doesn't return anything.

This could be done either in the compiler as part of warning waves (if that's still a thing in use?) or could it be an analyzer.

Start by reading src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerExecutor.cs around lines 1232-1236 and compare the compiler warning-wave and analyzer options raised in the issue. Determine which implementation path is appropriate and identify the relevant diagnostic tests; done means void methods using a XML comment produce the intended warning without affecting methods that return a value.

Written by the indexing model from the issue text.