[SeanKilleen]

The UnhandledExceptionHandling page existed but had two inaccuracies and a gap, checked against NUnitTestAssemblyRunner, UnhandledExceptionHandlingAttribute and the framework's own tests:

• Precedence was wrong. The page said settings at different levels are "combined - each level's configuration is additive". GetUnhandledExceptionHandlingFor actually walks method → fixture → assembly and uses the first level with a match; within a level, an entry naming the exception type beats an all-exceptions entry. Replaced with a "Precedence When Applied at Several Levels" section and an example of ignoring everything except one type at the same level.
• Platform behaviour was missing. On .NET Core / .NET 5+, an unhandled exception on another thread terminates the process regardless of the attribute (the framework tests are limited to NETFRAMEWORK for this reason), and unobserved Task exceptions are raised on the finalizer thread where NUnit cannot attribute them to a test. Added a "Platform Support" section and rewrote the "Common Scenarios" that used Task.Run as if the attribute would catch them.
• Added the constructor table, a "When to Use It" section (the issue asked when it is appropriate), fixed the See Also links, and added the missing row on the attributes index page.
• Updated the two snippet examples that spawned tasks to use threads that do not throw, with comments explaining why they cannot throw on modern .NET. Snippet tests pass (6 passed, 1 skipped on .NET as before).

Fixes #1151

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