Bug 1536224: Update Debugger documentation for async function initial yields. DONTBUILD r=jorendorff

Since bug 1530324 removed the wrapper functions for async functions, it is no
longer true that a Debugger.Frame for an async function call calls its onPop
handler once after evaluating default arguments but before beginning execution
of the body.

Differential Revision: https://phabricator.services.mozilla.com/D23939

--HG--
extra : moz-landing-system : lando

js/src/doc/Debugger/Debugger.Frame.md

@@ -153,15 +153,6 @@ If the caller then uses the generator's `.next()` method, which may or
 may not happen right away depending on the debuggee code, the suspended
 generator will be resumed, firing `.onEnterFrame` again.
 
-**Stepping into async functions** — SpiderMonkey also performs an
-initial yield for async functions. This is not so easy to defend by
-citing the spec; we're just exposing internal SpiderMonkey
-implementation details. When the initial yield happens, you'll observe
-an extra `.onPop` returning a generator object that SpiderMonkey will
-use internally, followed immediately by a second `.onEnterFrame` event
-for the same frame.
-
-
 ## Accessor Properties of the Debugger.Frame Prototype Object
 
 A `Debugger.Frame` instance inherits the following accessor properties from