[PATCH] doc: mention that ls time ordering is newest first

[Pádraig Brady]

FYI, in ls.c::usage() (and consequently the man page)
I'm adding this non obvious info

-                               otherwise: sort by ctime\n\
+                               otherwise: sort by ctime, newest first\n\

-  -t                         sort by modification time\n\
+  -t                         sort by modification time, newest first\n\