Add documentation on 'noBacktrack' preference

Subcommand parsers are often disjoint, meaning that if one fails
to parse after the command name then all others (having distinct
command names) will also fail to parse. When the 'noBacktrack'
preference is not set but 'showHelpOnError' is this leads to
the help for the top-level parser being shown instead of the
help for the failing subcommand.

A note has been added to the Haddock docs for the 'noBacktrack'
option to make this clear.

Options/Applicative/Builder.hs

@@ -484,6 +484,11 @@ showHelpOnEmpty :: PrefsMod
 showHelpOnEmpty = PrefsMod $ \p -> p { prefShowHelpOnEmpty = True }
 
 -- | Turn off backtracking after subcommand is parsed.
+--
+--    When used in conjunction with 'showHelpOnError' and a parser
+--    with subcommands it will cause the program, on error, to show
+--    the help for the subcommand instead of backtracking to the
+--    parent parser and trying to find an alternate parse there.
 noBacktrack :: PrefsMod
 noBacktrack = PrefsMod $ \p -> p { prefBacktrack = False }