Merge pull request #85126 from amartini51/task_cancellation_162979796

Document withTaskCancellationHandler parameters and at-most-once semantics

Fixes: rdar://162979796

Author: amartini51

stdlib/public/Concurrency/TaskCancellation.swift

@@ -17,6 +17,13 @@ import Swift
 /// Execute an operation with a cancellation handler that's immediately
 /// invoked if the current task is canceled.
 ///
+/// - Parameters:
+///   - operation: The operation to perform.
+///   - handler: A closure to execute on cancellation.
+///     If the task is canceled, this closure is called at most once;
+///     otherwise, it isn't called.
+///   - isolation: The actor that the operation and cancellation handler are isolated to.
+///
 /// This differs from the operation cooperatively checking for cancellation
 /// and reacting to it in that the cancellation handler is _always_ and
 /// _immediately_ invoked when the task is canceled. For example, even if the