(Standards conformance): Do not mention head -10,

(Standards conformance): Do not mention head -10,
since it now works the same regardless of POSIX version.
(od invocation): -w N -> -w[N].
(pr invocation): -S STRING -> -SSTRING.
(fold invocation): -WIDTH works even when conforming to POSIX
1003.1-2001.
(head invocation, tail invocation): Likewise for -NUM.
(split invocation): Likewise for -LINES.
(uniq invocation): Likewise for -N.
(expand invocation, unexpand invocation): Likewise for -TAB.
(nice invocation): Likewise for -ADJUSTMENT.
(sort invocation): Clarify explanation of +N option.
(uniq invocation): Likewise.
(join invocation): Remove special case for --help, --version.
(touch invocation): Clarify explanation of date options.
(Options for date): -I timespec -> -I[timespec].