Clarify task types (#875)

* Clarify task types

- `Task` and `Task<T>` are classified as task types despite not
  specifying builder types
- Interfaces are allowed to be task types
- Enums and methods are prohibited from being decorated with
  AsyncMethodBuilderAttribute
- Task types must not be generic in more than one type parameter
  (including in terms of containing types)

Discussion required for all of this, but if merged, would
fix #854, #856, #858 and #859.

* Prevent nesting within generic types, for task types.

This wording follows the wording in 15.15.2 for task builders.

standard/classes.md

@@ -5207,8 +5207,7 @@ A method ([§15.6](classes.md#156-methods)) or anonymous function ([§12.19](exp
 
 It is a compile-time error for the formal parameter list of an async function to specify any `in`, `out`, or `ref` parameters, or any parameter of a `ref struct` type.
 
-The *return_type* of an async method shall be either `void` or a ***task type***. For an async method that returns a value, a task type shall be generic. For an async method that does not return a value, a task type shall not be generic. Such types are referred to in this specification as `«TaskType»<T>` and `«TaskType»`, respectively. (The Standard library type `System.Threading.Tasks.Task` and types constructed from `System.Threading.Tasks.Task<TResult>` are task types.)
-A task type shall be a class or struct type that is associated with a ***task builder type*** via the attribute `System.Runtime.CompilerServices.AsyncMethodBuilderAttribute`. Such types are referred to in this specification as `«TaskBuilderType»<T>` and `«TaskBuilderType»`.
+The *return_type* of an async method shall be either `void` or a ***task type***. For an async method that returns a value, a task type shall be generic. For an async method that does not return a value, a task type shall not be generic. Such types are referred to in this specification as `«TaskType»<T>` and `«TaskType»`, respectively. The Standard library type `System.Threading.Tasks.Task` and types constructed from `System.Threading.Tasks.Task<TResult>` are task types, as well as a class, struct or interface type that is associated with a ***task builder type*** via the attribute `System.Runtime.CompilerServices.AsyncMethodBuilderAttribute`. Such types are referred to in this specification as `«TaskBuilderType»<T>` and `«TaskBuilderType»`. A task type can have at most one type parameter and cannot be nested in a generic type.
 
 An async method returning a task type is said to be ***task-returning***.
 

standard/standard-library.md

@@ -463,8 +463,7 @@ namespace System.Linq.Expressions
 namespace System.Runtime.CompilerServices
 {
     [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | 
-        AttributeTargets.Interface | AttributeTargets.Delegate | 
-        AttributeTargets.Enum | AttributeTargets.Method, 
+        AttributeTargets.Interface, 
         Inherited = false, AllowMultiple = false)]
     public sealed class AsyncMethodBuilderAttribute : Attribute
     {