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

[Jie Fu]

Hi David and Erik,

Thank you for your review and very nice suggestions.
Here is the latest version: 
http://cr.openjdk.java.net/~jiefu/8221894/webrev.03/
Please see comments inline and review.
Thanks a lot.

> I have two comments on the proposed text:
>
> 1. I think "oraclelinux" should be "Oracle Linux".
Done.
>
> 2. I don't understand the example as I don't know why Ubuntu 16.04 
> would be considered something "not newer than" OL 7.6. I'm guessing 
> this relates to the kernel version in each ?? If so perhaps we should 
> spell out what the relevant kernel version is?
Good question. It relates to the glibc version in each.
Systems with glibc versions not compatible with the one used in the 
default docker image (e.g., Oracle Linux 7.6 for x86) will fail with 
default test parameters.
Done.

Best regards,
Jie