readlink: improve documentation for POSIXLY_CORRECT in --help

readlink: improve documentation for POSIXLY_CORRECT in --help

* NEWS: Improve wording to not imply POSIXLY_CORRECT=1 readlink -q will
be verbose.
* src/readlink.c (usage): Mention the affect of POSIXLY_CORRECT on
-s (--silent), -q (--quiet), and -v (--verbose) in the help message.
(main): Remove spurious newline added by previous commit.
* doc/coreutils.texi (readlink invocation): Mention that -s (--silent)
and -q (--quiet) are not the default when POSIXLY_CORRECT is set.