[doc] Information on JVM fork count

I spent 20 minutes reading gradle docs to figure out how to do this. No one
else should have to do that.

Also, some of the documentation was out of date.

TESTING.asciidoc

@@ -149,17 +149,23 @@ gradle test -Dtests.awaitsfix=[false] - known issue (@AwaitsFix)
 
 === Load balancing and caches.
 
-By default, the tests run sequentially on a single forked JVM.
-
-To run with more forked JVMs than the default use:
+By default the tests run on up to 4 JVMs based on the number of cores. If you
+want to explicitly specify the number of JVMs you can do so on the command
+line:
 
 ----------------------------
 gradle test -Dtests.jvms=8
 ----------------------------
 
-Don't count hypercores for CPU-intense tests and leave some slack
-for JVM-internal threads (like the garbage collector). Make sure there is
-enough RAM to handle child JVMs.
+Or in `~/.gradle/gradle.properties`:
+
+----------------------------
+systemProp.tests.jvms=8
+----------------------------
+
+Its difficult to pick the "right" number here. Hypercores don't count for CPU
+intensive tests and you should leave some slack for JVM-interal threads like
+the garbage collector. And you have to have enough RAM to handle each JVM.
 
 === Test compatibility.