(allout-expose-topic): Rectify implementation of "+" spec, so that bodies are

(allout-expose-topic): Rectify implementation of "+" spec, so that bodies are
not exposed with headlines.
(allout-current-depth): Do aberrent check only at or below doublecheck depth.
(allout-chart-subtree): Make it explicit that LEVELS being nil means unlimited
depth.  Drop undocumented support for LEVELS value t meaning unlimited depth.
(This is consistent with allout-chart-to-reveal, but contrary to
allout-show-children, which needs to use nil to default to depth of 1.)
(allout-goto-prefix-doublechecked): Wrap long docstring line.
(allout-chart-to-reveal): Be explicit in docstring about meaning of nil LEVELS,
and drop support for LEVELS value t.
(allout-show-children): Translate the level spec used by this routine to that
used by allout-chart-subtree and allout-chart-to-reveal.
(allout-show-to-offshoot): Retry once when stuck, after opening subtree -
improvements in discontinuity handling likely will enable progress.