Allow AsyncMethodBuilder() attribute on methods

[BillWagner]

This is an advanced scenario that enables library authors to optimize away allocations on async method.

See dotnet/csharplang#1407

[BillWagner]

There are a few tasks here for this issue:

1. The General attributes read by the compiler article does not include AsyncMethodBuilder. That was an oversight from when generalized async return types was added. The generalized async return types should add a link to this update.
2. Both the attributes read, and the generalized async return types should include information on the new behavior for the attribute.
3. Create a new issue for an article in the advance section that covers:

• How to use the pooling builder that will be provided in .NET 6, both for different types and for individual methods
• How to create a builder.

The last task is a separate task because of the relatively small audience. It can wait until later in the release cycle.