RFR: JDK-8219906: Update test documentation with default test jobs settings

[Ao Qi]

Hi,

Test jobs setting for sparc is different with x86 (JDK-8211727).
Besides, test jobs setting is based on memory size (JDK-8214003). The
test documentation fails to mention these and needs update.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219906

Webrev: http://cr.openjdk.java.net/~aoqi/8219906/webrev.00/

Cheers,
Ao Qi