Clarify description of FEATURES in control-spec.
(1) On by default is a bad way to describe features. Rather, they
are always on and should be viewed as a part of the control
protocol. Updated the wording in USEFEATURE to reflect this.
(2) Made descriptions of Tor versions consistant across all
features. There is the version in which a feature was introduced and
the version in which it became part of the protocol.
(3) Reworded the description of the VERBOSE_NAMES feature. The
previous wording describes the way things used to be first. Better to
lead with the current state of things and then describe how it differs
from old versions.