Please clarify "Inside of an async function" means not just the immediately enclosing

See #25448 for context.

From [the spec](https://github.com/dotnet/csharplang/blob/7f39331672cf8edbda8867de004138e0f711c877/spec/expressions.md#await-expressions):

> Inside of an async function, `await` cannot be used as an identifier

A non-async local function that is inside an async method is still "inside" of an async function, and therefore `await` is a keyword there.

It would be nice to clarify that this does not apply just to the immediately enclosing function. If *any* enclosing function is async, then `await` is a keyword.