RFR: JDK-8217634 RunTest documentation and usability update

[Magnus Ihse Bursie]

The RunTest documentation (doc/testing.md) has not been properly updated 
with the latest changes.

Also, some newly added keywords were not classified as STRING, but as 
SINGLE to ParseKeywordVariable, which means that the user would not have 
the quoting support that should have been available.

I also added in some whitespace fixes, for good measure.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217634
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8217634-runtest-love-and-care/webrev.01

/Magnus