Improve ConditionalAttribute documentation

[svick]

I think ConditionalAttribute is one of the more confusing features of C#/.Net, so it's important that it's well documented.

Some of the ways in which the document could be improved:

• Make sure it's clear that when the C# compiler removes a call to a method, it also removes its receiver and parameters, which can have side-effects. See ConditionalAttribute removes too much from the call site roslyn#28801. This should include updating both remarks and the example.
• Clarify the description of the example: it says it assumes using /define, but then the code has #define.
• Improve the example, so that ConditionalAttribute actually affects its output.
• Either make the list of ways of specifying conditional compilation symbols include the two that I think are most commonly used (use the VS IDE, edit .Net Core SDK .csproj) or link to an article that does.
• The article says that you can set conditional compilation symbols though environment variables. Is that really true?
• Remove mention of JScript, add F#.
• I don't understand the note about VB and AddressOf. If it's saying something useful, it should be clarified.

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Document Details

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• ID: 0380b9c1-45c7-2cad-4111-c3cb124d0663
• Version Independent ID: d2b35193-b11f-2cd9-7377-13c57c3f25a7
• Content: ConditionalAttribute Class (System.Diagnostics)
• Content Source: xml/System.Diagnostics/ConditionalAttribute.xml
• Product: .net
• GitHub Login: @mairaw
• Microsoft Alias: mairaw

I'd like to add that the AttributeUsage attribute appears two times in the definition of the ConditionalAttribute.

We should change

[System.AttributeUsage(System.AttributeTargets.Class | System.AttributeTargets.Method, AllowMultiple=true)]
[System.Runtime.InteropServices.ComVisible(true)]
[System.AttributeUsage(System.AttributeTargets.Method, AllowMultiple=true)]
public sealed class ConditionalAttribute : Attribute

to

[System.AttributeUsage(System.AttributeTargets.Class | System.AttributeTargets.Method, AllowMultiple=true)]
[System.Runtime.InteropServices.ComVisible(true)]
public sealed class ConditionalAttribute : Attribute

[svick]

@johnnyagerard I think that's a separate issue that is already tracked at dotnet/docs#6971.

[mairaw]

@wtgodbe, @krwq can you take a look at this feedback for the docs for ConditionalAttribute? Thanks.

[krwq]

I agree with the assessment.