(Exit status): Document that ordinary failure

(Exit status): Document that ordinary failure
might not exit with status 1 on unusual platforms.
Mention chroot, env, nice, and su as having unusual exit
status patterns.  Don't bother to mention true and false
since their exit status patterns are actually normal.
(sort invocation, su invocation): Mention its unusual exit
status pattern.
(chroot invocation): Simplify description of exit status 1.
Remove duplicate description of status 127.
(env invocation): Use consistent tenses; simplifiy description
of status 1.
(nice invocation): Likewise.