examples of focusable area should include tabindex=-1 (not just tabindex >= 0)

[carmacleod]

The table of examples in the definition of focusable area has this phrase:

the element's tabindex value is a non-negative integer

Unless I have misunderstood the purpose of this table, I think it should instead say:

the element's tabindex value is an integer

or possibly:

the element's tabindex value is non-null

Please let me know if this is correct (and if so, which new phrase is preferred), and I will provide a PR.

[domenic]

@rakina @tkent-google were touching this table last. I think non-null is more correct here, but it is a bit subtle, so I'd love a double-check from one of them...

[rakina]

Oh oops... yeah it should be non-null instead. Thanks for spotting!