Improve habit documentation.

Patch by Robert Horn.
TINYCHANGE

doc/org.texi

@@ -3794,7 +3794,10 @@ The habit is a TODO, with a TODO keyword representing an open state.
 @item
 The property @code{STYLE} is set to the value @code{habit}.
 @item
-The TODO has a scheduled date, with a @code{.+} style repeat interval.
+The TODO has a scheduled date, usually with a @code{.+} style repeat
+interval.  A @code{++} style may be appropriate for habits with time
+constraints, e.g., must be done on weekends, or a @code{+} style for an
+unusual habit that can have a backlog, e.g., weekly reports.
 @item
 The TODO may also have minimum and maximum ranges specified by using the
 syntax @samp{.+2d/3d}, which says that you want to do the task at least every