[heycam/webidl] [upon fulfillment, upon rejection] reword (#923)

In **upon fulfillment** https://heycam.github.io/webidl/#upon-fulfillment this part seems incorrect and confusing: 
![image](https://user-images.githubusercontent.com/19698636/93589233-d0250680-f9ac-11ea-898f-bc68684fee0e.png)

When the `promise` is fulfilled with the value `v` the `onFulfilled` gets called with `v`. This is not the same as performing steps with `v`. `onFulfilled` is produced using all the steps in 1. and 2. of https://heycam.github.io/webidl/#dfn-perform-steps-once-promise-is-settled

I guess the intention of **upon fulfillment** was:

> 1. **React** to _promise_ providing _steps_ as the steps to be run when the promise is fulfilled.

So I think it should be changed into this.

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NOTE 1: If this is correct, and when doing this required change, it becomes kind of reasonable to remove **upon fulfillment** completely and to always call **react** directly.

NOTE 2: Same discussion applies to **upon rejection**

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