Warning fixes

[MrJul]

What does the pull request do?

Another batch of warning fixes:

• Fixes XML doc warnings (again)
• Fixes CA1854: use Dictionary.TryGetValue instead of ContainsKey+indexer
• Fixes CS9123: avoid & in async methods
• Use SetCurrentValue to set DataGridColumn.Width

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0045994-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[kekekeks]

Should probably use span casts instead.

[MrJul]

It was actually what I tried first, but I found the resulting code not buying much:

MemoryMarshal.AsBytes(new Span<WebSocketHeader>(&source, 1))
    .Slice(0, headerLength)
    .CopyTo(destination);

Not even saving the pointer usage to be compatible with .NET Standard.
That's basically the implementation of Marshal.Copy, so I didn't bother.

If the struct didn't have a variable size, we could simply use MemoryMarshal.Write(destination, ref header) instead: clear and concise. At that point, since we need 3 different structs, we're probably better writing directly to the destination buffer using BinaryPrimitives.WriteX().