Simple change in juju set to use the already written documentation

A variable named setDoc with the documentation already existed a longer time. But it never has been used as command documentation.

Fixes https://bugs.launchpad.net/juju-core/+bug/1359187
Looking at this now, I realize that this is slightly bogus. While it does mean that the latter tests are asserting that "set" is actually integrated, it would have passed without your patch. So this test doesn't actually cover what we want it to (that setDoc is *actually* the help text we are getting.)

Any thoughts on how to test this? Some people don't like repeating the verbatim test in the test suite, but I find it helps to have 2 passes to prevent accidental X==X sort of tests.