Change the default number of threads

Following @jnthn advice. Closes #1065, maybe for good this time.

Author: JJ

doc/Language/variables.pod6

@@ -1289,12 +1289,12 @@ L<Telemetry> has been loaded.
 
 Note on usage of C<$*SCHEDULER>:
 
-For the current Rakudo, by default this imposes a maximum of 16 threads on the methods
-C<.hyper> and C<.race>, and other thread-pool classes that use that scheduler such as C<Promise>s or C<Supplie>s. To change the maximum number of threads, either set the
+For the current (2018.04) Rakudo, by default this imposes a maximum of 64 threads on the methods
+C<.hyper>, C<.race> and other thread-pool classes that use that scheduler such as C<Promise>s or C<Supplie>s. To change the maximum number of threads, you can either set the
 environment variable C<RAKUDO_MAX_THREADS> before running perl6 or create a scoped copy
 with the default changed before using them:
 
-    my $*SCHEDULER = ThreadPoolScheduler.new( max_threads => 64 );
+    my $*SCHEDULER = ThreadPoolScheduler.new( max_threads => 128 );
 
 This behavior is not tested in the spec tests and is subject to change.