Draft: Resolve "Windows Runners: Document "session 0" restrictions (screen resolution statically set to 1024x768)"

What does this MR do?

Documents the "session 0" screen resolution limitation.

Why was this MR needed?

It wasn't immediately clear as to why the resolution was limited. To avoid future confusion, let's have it documented ☺

What's the best way to test this MR?

Run the following PowerShell script on a Windows Runner:

echo "Current session:"
[System.Diagnostics.Process]::GetCurrentProcess().SessionId

Add-Type -AssemblyName System.Windows.Forms
[System.Windows.Forms.Screen]::AllScreens

Observe correlation between SessionId and screen resolution.

What are the relevant issue numbers?

Closes #37955