Documented this feature and integrated a newer version of Stapler.

I took the opportunity to reorder the content in the section, so that
the tree parameter becomes the main subject.

At this point, the depth parameter should be only used in a limited
circumstance, so it should be treated as such.

[FIXED JENKINS-22938] SSH slave connections die after the slave outputs 4MB of stderr, usually during findbugs analysis

This lets us support plugins written in Java8

Integrated v2.38