Update description of emphasis parsing algorithm in spec.

See commonmark/cmark#383.

spec.txt

@@ -9706,8 +9706,7 @@ just above `stack_bottom` (or the first element if `stack_bottom`
 is NULL).
 
 We keep track of the `openers_bottom` for each delimiter
-type (`*`, `_`) and each length of the closing delimiter run
-(modulo 3).  Initialize this to `stack_bottom`.
+type (`*`, `_`). Initialize this to `stack_bottom`.
 
 Then we repeat the following until we run out of potential
 closers:
@@ -9741,9 +9740,13 @@ closers:
 
 - If none is found:
 
-  + Set `openers_bottom` to the element before `current_position`.
-    (We know that there are no openers for this kind of closer up to and
-    including this point, so this puts a lower bound on future searches.)
+  + Set `openers_bottom` for this delimiter type to the element
+    before the first potential opener that was rejected solely
+    because of the "multiple of 3" rule, or to the element
+    before `current_position`, if there are no such potential
+    openers.  (We know that there are no openers for this kind
+    of closer up to and including this point, so this puts a
+    lower bound on future searches.)
 
   + If the closer at `current_position` is not a potential opener,
     remove it from the delimiter stack (since we know it can't