Add doc comments to all C.3 types

On the 2026-05-06 TG2 call, we decided to add documentation comments to all C.3 types in the same way as Nigel did for `Index` and `Range` in V8. (Nigel took the comments from the Roslyn source code, modifying them as needed.)

Below is a list of all those types, from V5 through V13. (**V13 entries are under construction.**)

- There are 10 V5 types needing doc comments.
- There are 1 V6 types needing doc comments.
- There are 8 V7 types needing doc comments.
- There are 19 V8 types needing doc comments.
- There are 14 V9 types needing doc comments.
- There are 4 V10 types needing doc comments.
- There are 4 V11 types needing doc comments.
- There are 3 V12 types needing doc comments.
- There are 2 V13 types needing doc comments.

## `namespace System`

- [] V8: `interface IAsyncDisposable`
- [] V7: `class FormattableString`
- [] V11: `class MemoryExtensions`
- [] V9: `class OperationCanceledException`
- [x] V8: `struct Index`
- [x] V8: `struct Range`
- [] V7: `struct ReadOnlySpan`
- [] V7: `struct Span`
- [] V8: `struct ValueTuple`
- [] V8: `struct ValueTuple`
- [] V8: `struct ValueTuple`
- [] V8: `struct ValueTuple`
- [] V8: `struct ValueTuple`
- [] V8: `struct ValueTuple`
- [] V8: `struct ValueTuple`
- [] V8: `struct ValueTuple`

## `namespace System.Collections.Generic`

- [] V8: `interface IReadOnlyCollection`
- [] V8: `interface IReadOnlyList`
- [] V8: `interface IAsyncEnumerable`
- [] V8: `interface IAsyncEnumerator`

## `namespace System.Diagnostics.CodeAnalysis`

- [] V9: `class AllowNullAttribute`
- [] V9: `class DisallowNullAttribute`
- [] V9: `class DoesNotReturnAttribute`
- [] V9: `class DoesNotReturnIfAttribute`
- [] V9: `class MaybeNullAttribute`
- [] V9: `class MaybeNullWhenAttribute`
- [] V9: `class MemberNotNullAttribute`
- [] V9: `class MemberNotNullWhenAttribute`
- [] V9: `class NotNullAttribute`
- [] V9: `class NotNullIfNotNullAttribute`
- [] V9: `class NotNullWhenAttribute`
- [] V11: `class UnscopedRefAttribute`
- [] V11: `class SetsRequiredMembersAttribute`

## `namespace System.Linq.Expressions`

- [] V5: `class Expression`

## `namespace System.Runtime.CompilerServices`

- [] V12: `class CollectionBuilderAttribute`
- [] V10: `struct DefaultInterpolatedStringHandler`
- [] V7: `class AsyncMethodBuilderAttribute`
- [] V10: `class CallerArgumentExpressionAttribute`
- [] V5: `class CallerFilePathAttribute`
- [] V5: `class CallerLineNumberAttribute`
- [] V5: `class CallerMemberNameAttribute`
- [] V8: `class EnumeratorCancellationAttribute`
- [] V6: `class FormattableStringFactory`
- [] V12: `class InlineArrayAttribute`
- [] V5: `interface ICriticalNotifyCompletion`
- [] V5: `interface INotifyCompletion`
- [] V10: `class InterpolatedStringHandlerArgumentAttribute`
- [] V10: `class InterpolatedStringHandlerAttribute`
- [x] V8: `interface ITuple`
- [] V9: `class ModuleInitializerAttribute`
- [] V13: `class OverloadResolutionPriorityAttribute`
- [] V11: `class RequiredMemberAttribute`
- [] V5: `struct TaskAwaiter`
- [] V5: `struct TaskAwaiter`
- [] V9: `class UnmanagedCallersOnlyAttribute`
- [] V7: `struct ValueTaskAwaiter`
- [] V7: `struct ValueTaskAwaiter`

## `namespace System.Threading`

- [] V8: `class CancellationTokenSource`
- [] V8: `struct CancellationToken`
- [] V13: `class Lock`

## `namespace System.Threading.Tasks`

- [] V5: `class Task`
- [] V5: `class Task`
- [] V7: `struct ValueTask`
- [] V7: `struct ValueTask`