Documenting InvalidOperationException for ReadLockAsync (and possibly other methods)

[MartyIX]

I know that AsyncReaderWriterLock has the "completion" feature (i.e. asyncReaderWriterLock.Complete()) but:

vs-threading/src/Microsoft.VisualStudio.Threading/AsyncReaderWriterLock.cs

Lines 408 to 416 in 1738f83

	/// <summary>
	/// Obtains a read lock, asynchronously awaiting for the lock if it is not immediately available.
	/// </summary>
	/// <param name="cancellationToken">
	/// A token whose cancellation indicates lost interest in obtaining the lock.
	/// A canceled token does not release a lock that has already been issued. But if the lock isn't immediately available,
	/// a canceled token will cause the code that is waiting for the lock to resume with an <see cref="OperationCanceledException"/>.
	/// </param>
	/// <returns>An awaitable object whose result is the lock releaser.</returns>

is not really helpful in explaining what happens when the AsyncReaderWriterLock.Complete() has been called.

For me it throws: System.InvalidOperationException : This lock has already been marked for completion. No new top-level locks can be serviced.

Would it be worth documening that?

cc @AArnott

[AArnott]

Sure. Are you thinking some /// <exception docs on the impacted APIs? If so, would you be interested in sending a PR?