Correct OPf_SPECIAL/OP_UNSTACK comment

I misread the code when I added it.
[perl #123286] Lone C-style for in a block

A block in perl usually consists of an enter/leave pair plus the con-
tents of the block:

  leave
     enter
     nextstate
     whatever

But if the contents of the block are simple enough to forego the
full block structure, a simple scope op is used, which is not
even executed:

  scope
    ex-nextstate
    whatever

If there is a real nextstate op anywhere in the block, it resets the
stack to whatever it was at block entry, based on the value on the
context stack placed there by the enter op.  That’s why we can never
have scope+nextstate (we have ex-nextstate, or a former nextstate op
that is not executed).

A for-loop (for(init; cond; cont) { ... }) executes the init section
first, and then an unstack op, which is like nextstate in that it
resets the stack based on what the context stack says is the base off-
set for this block.

If we have an unstack op, we can’t use scope, just as we can’t use it
with nextstate.  But we *were* nonetheless using scope in this case.

Hence, map { for(...;...;...) {...} } ... caused the for-loop to reset
the stack to the beginning of map’s own arguments.  So the for-loop
would stomp on them.

We can see the same bug with ‘for’ clobbering an outer list:

$ perl5.20.1 -le 'print 1..3, do{for(0;0;){}}, 4..6;'
0456