(Mode Structure): Modernize the explanation of the

(Mode Structure): Modernize the explanation of the
setuid and setgid bits on directories.
(Changing Special Mode Bits): Mention that a implies both u and g
for s.  Cross reference to new node.
(Numeric Modes): Don't claim that 0055 is the same as 55; this isn't
true any more.  Mention new node.
4755 is now like u=rwxs,go=rx,g-s, not like u=rwxs,go=rx.
(Directory Setuid and Setgid): New node.