(Mode Structure, Changing Special Permissions):

(Mode Structure, Changing Special Permissions):
(Conditional Executability, Numeric Modes):
These days the sticky bit is more often uses as the restricted
deletion flag, so modernize the discussion about this.
(Mode Structure): Linux/GNU -> GNU/Linux.
(Symbolic Modes): Don't imply that "+ur" or "u" is valid.
(Setting Permissions): Don't imply that "+t" is invalid.
Use "rwx" rather than the less-common "rxw" in an example.
(Copying Permissions): Say that ugo is a replacement for
a string of the other letters.  Add spaces around examples.
Use "set-user-ID" rather than "set user ID" to avoid ambiguity.
Use "+t" rather than "o+t", since POSIX doesn't specify the latter.
Mention which combinations are portable and which are GNU.
(Numeric Modes): Don't imply they aren't portable; they are
nowadays.