Revise emphasis parsing algorithm description...

to fit most recent fix for #383.

Author: jgm

spec.txt

@@ -9706,8 +9706,9 @@ 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 (`*`, `_`), indexed to the length of the closing delimiter run
+(modulo 3) and to whether the closing delimiter can also be an
+opener.  Initialize this to `stack_bottom`.
 
 Then we repeat the following until we run out of potential
 closers: