PlatformAttribute has AllowMultiple = true, but that doesn't work.

[manfred-brands]

Found whilst working on #3926

On a test with more than one attribute:

[Platform(Include = "Linux")]
[Platform(Include = "Win")]

I do get the error: Only supported on Linux when running under Windows.

Yes, the documentation mentions combining the platforms using commas, but should we make this a compile error?

Should the AttributeUsage be update to AllowMultiple = false?

[rprouse]

One of the problems with our platform attribute is that it is used for multiple things, so we could have

[Platform(Include = "Linux")]
[Platform(Include = "64-Bit")]

[stevenaw]

If multiple is desired, then I think @manfred-brands fixed it in main via #3926
Should the relevant parts of that PR be backported to 3.13.x in order to close this?

[stevenaw]

I am going to close this as fixed in 4.0 as it was done in #3926 based on this comment: #3926 (comment)

It doesn't look like we want to backport it but any future effort there is to be tracked under a separate issue.

I'll put up a PR against the docs repo shortly to ensure this is in the 4.0 release notes as well