bug#34009: warn that mkdir --mode doesn't affect parents created

[積丹尼 Dan Jacobson]

On man mkdir

       -m, --mode=MODE
              set file mode (as in chmod), not a=rwx - umask

       -p, --parents
              no error if existing, make parent directories as needed

do warn that --mode doesn't affect any parents created.

$ mkdir --mode 700 -p /tmp/g/h/i
$ find /tmp/g -ls
    55795      0 drwxr-xr-x   3 jidanni  jidanni        60 Jan  7 23:30 /tmp/g
    55796      0 drwxr-xr-x   3 jidanni  jidanni        60 Jan  7 23:30 /tmp/g/h
    55797      0 drwx------   2 jidanni  jidanni        40 Jan  7 23:30 
/tmp/g/h/i

Also warn on (info "(coreutils) mkdir invocation") more directly. Thanks.