Fibonacci regex, fastest: Fix an erroneous comment, and add a new comment
Fibonacci regex, fastest: Apply length+speed optimizations; length-=39

These are shorter constructions which are currently optimized under RegexMathEngine. As it turns out they're also optimized under Perl and PCRE, it seems (at least, in a test I did up to 17711).
Fibonacci regex, fastest: Simplify condition for exiting loop; length-=17