less: document -S flag and make it independently configurable

less: document -S flag and make it independently configurable

The -S flag, to cause long lines to be truncated, was enabled by
FEATURE_LESS_DASHCMD.  This is non-obvious and -S is useful even
if the '-' command isn't enabled.

function                                             old     new   delta
.rodata                                           156045  156077     +32
packed_usage                                       30223   30244     +21
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 53/0)               Total: 53 bytes

Signed-off-by: Ron Yorston <rmy@frippery.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>