RFR: 8221894: Add comments for docker tests in the test doc

[Jie Fu]

Hi Erik,

Thank you for your review.

> Hello Jie,
>
> I think this kind of information would fit better under its own new 
> heading to make it more explicit. Then we could continue filling in 
> other similar notes for other tests there. At the bottom, something like:
>
> ## Notes for Specific Tests
>
> ### Docker Tests
>
> <Add basically what you put in the review email here>
>
>
> What do you think?

Very good suggestions.
Here is the updated version: 
http://cr.openjdk.java.net/~jiefu/8221894/webrev.01/
Please review.
Thanks a lot.

Best regards,
Jie