(maint) document max-threads

This documents the Jetty setting for 'max-threads'.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #608 from kbarber/maint/1.1.x/document-max-threads

(maint) document max-threads
Merge branch '1.1.x' into 1.2.x

* 1.1.x:
  (maint) document max-threads