The member keys in an xml doc file depend on the compiler that wrote it
(C# vs the MSVC C++/CLI dialect), so XmlDocumentationProvider's
entity-based lookup now tries each candidate form in order. The tooltip
path goes through the entity overload instead of building the key
itself, so the fallback lives in one place. The C++/CLI form is tried
first: wherever it differs it contains character sequences Roslyn never
writes, so it can only match MSVC-generated keys, while the stripped
Roslyn form of one member could match the key of a different member in
an MSVC-generated file.
Assisted-by: Claude:claude-fable-5:Claude Code
Promotes the modern .NET XML-doc lookup from the Avalonia port into the
shared ICSharpCode.Decompiler library so every host (WPF, Avalonia, any
third-party consumer of XmlDocLoader) gets hover/tooltip documentation
for system entities without per-host fallback wiring.