Be clearer about the underlying source of a constructed ReadableStream.

Author: jyasskin

fetch.bs

@@ -1878,11 +1878,11 @@ are optional, run these steps:
 <ol>
  <li><p>Let <var>init</var> be a new object.
 
- <li><p>Set <var>init</var>["pull"] to a function that runs <var>pull</var> if <var>pull</var> is
- given.
+ <li><p>If <var>pull</var> is given, set <var>init</var>["pull"] to a function that returns
+ <var>pull</var>().
 
- <li><p>Set <var>init</var>["cancel"] to a function that runs <var>cancel</var> if
- <var>cancel</var> is given.
+ <li><p>If <var>cancel</var> is given, set <var>init</var>["cancel"] to a function taking a
+ <var>reason</var> that returns <var>cancel</var>(<var>reason</var>).
 
  <li><p>Let <var>stream</var> be the result of calling the initial value of {{ReadableStream}} as
  constructor with <var>init</var> and <var>strategy</var> if given.