Update for habit documentation

[Robert Horn <rjhorn@panix.com>]

I've been using habits and found additional uses that are actually
implemented but not mentioned in the documentation.  What I've found
useful are:

1) Using a '++' repeat style for habits that have calendar constraints.
 I use this for some activities that must be done on Monday, or on
weekends.  This works, so there is no need to change the implementation.

2) Using a '+' repeat style is good for activities that can have a
backlog.  For example, preparing weekly reports is something that is
aided by the habit tracking information.  This also works.

This affects bullet 4, which presently implies that the repeat interval
must be a '.+' style. Would the following be clear:

4. The TODO has a scheduled date, usually with a '.+' style repeat
interval.  Other repeat styles may be used.  The '++' style can handle
habits with calendar constraints, e.g., "must be done on a Monday"; and
the '+' style can be useful for habits that can have a backlog.

R Horn