ModelWriter.WriteExtensibleEnum can generate incorrectly-typed documentation

[Arithmomaniac]

See the following generator source:

autorest.csharp/src/AutoRest.CSharp/Common/Generation/Writers/ModelWriter.cs

Lines 432 to 433 in 5088d5b

	writer.WriteXmlDocumentationSummary($"Converts a string to a {name:C}.");
	writer.Line($"public static implicit operator {cs}({enumType.ValueType} value) => new {cs}(value);");

Note that even when enumType.ValueType is not string, the generated documentation still refers to a string. In the (rare) cases where it isn't a string, it should refer to the actual type (ideally, wrapped in a see cref tag.)