doc: mention QUOTING_STYLE env var in ls man page

* src/ls.c (usage): Mention QUOTING_STYLE with the --quoting-style
option, and indicate it has lower precedence than that option.

src/ls.c

@@ -5229,6 +5229,7 @@ Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.\n\
       --quoting-style=WORD   use quoting style WORD for entry names:\n\
                                literal, locale, shell, shell-always,\n\
                                shell-escape, shell-escape-always, c, escape\n\
+                               (overrides QUOTING_STYLE environment variable)\n\
 "), stdout);
       fputs (_("\
   -r, --reverse              reverse order while sorting\n\