Change NamedInstance.rndc() doctest into doc example

Change NamedInstance.rndc() doctest into doc example

The test is troublesome, because NamedInstance(identifier) expects that
a directory with such a name exists. While it'd be possible to mock
those directories as well, it'd make the doctest overly long and
complex, which isn't justified, given that it's only testing a couple of
options. Turn it into regular documentation instead.

(cherry picked from commit dcfb6c23da975427e73a21a77636b6f5afcfe5ac)