Normative: Ban |> await

Given problems with all possibilities considered for await integration
with pipeline, ban |> await within an async function as the initial
option. When we have considered things further, we could add await
integration as a follow-on feature.

Relates to tc39#66

README.md

@@ -78,30 +78,7 @@ As you can see, because the pipe operator always pipes a single result value, it
 
 ### Use of `await`
 
-The pipeline operator allows the result of a `Promise`-returning function to be `await`ed as follows:
-
-
-```js
-x |> await f
-```
-
-which is the equivalent of
-
-```js
-await f(x)
-```
-
-This is to allow you to `await` the result of an asynchronous function and pass it to the next function from within a function pipeline, as follows:
-
-```js
-const userAge = userId |> await fetchUserById |> getAgeFromUser
-```
-
-which is the equivalent of
-
-```js
-const userAge = getAgeFromUser(await fetchUserById(userId))
-```
+The pipeline operator does not have any special integration with async/await in its initial version, due to [issues](https://github.com/tc39/proposal-pipeline-operator/issues/66) with various alternatives discussed. To leave space for future additions to integrate await support, the not-very-useful sequence `|> await` (which would otherwise await a function, not the result of calling a function) is a SyntaxError.
 
 ### Usage with Function.prototype.papp
 

spec.html

@@ -36,7 +36,6 @@ <h1>Syntax</h1>
         LogicalORExpression[?In, ?Yield, ?Await]
         [~Await] PipelineExpression[?In, ?Yield, ?Await] `|>` LogicalORExpression[?In, ?Yield, ?Await]
         [+Await] PipelineExpression[?In, ?Yield, ?Await] `|>` [lookahead &lt;! {`await`}] LogicalORExpression[?In, ?Yield, ?Await]
-        [+Await] PipelineExpression[?In, ?Yield, ?Await] `|>` `await` LogicalORExpression[?In, ?Yield, ?Await]
     </ins>
   </emu-grammar>
 </emu-clause>
@@ -81,11 +80,6 @@ <h1>Runtime Semantics: Evaluation</h1>
         1. Let _result_ be the result of PipelineEvaluate for |PipelineExpression| and |LogicalORExpression|.
         1. Return _result_.
     </emu-alg>
-    <emu-grammar>PipelineExpression : PipelineExpression `|>` `await` LogicalORExpression</emu-grammar>
-    <emu-alg>
-        1. Let _result_ be the result of PipelineEvaluate for |PipelineExpression| and |LogicalORExpression|.
-        1. Return ? AsyncFunctionAwait(_result_).
-    </emu-alg>
     <emu-grammar>PipelineExpression : LogicalORExpression</emu-grammar>
     <emu-alg>
         1. Return the result of evaluating |LogicalORExpression|.