[css-counter-styles] Allow impls to extend range of hebrew

In the currently definition of hebrew counter style in the spec, there's a
comment says [1]:

This system theoretically extends to at least 999999, but specifying it
above 10999 with the @counter-style rule is difficult.

I suggest it also say that impls are encouraged to implement this style to
the extended range, just like the note in complex cjk states.

I bring this up because I see a testcase which tests 20000/20001 fails on
our impl [2], but AFAICS, the behavior of WebKit/Blink matches ours (except
that they don't reorder the marker text correctly.) And I think that test
should be rejected.

[1] http://dev.w3.org/csswg/css-counter-styles/#hebrew
[2]
http://www.w3.org/International/tests/repo/results/predefined-counter-styles#simplenumeric

- Xidorn