doc: warn about following symlinks recursively in chown/chgrp

doc: warn about following symlinks recursively in chown/chgrp

In both chown and chgrp (which shares its code with chown), operating
on symlinks recursively has a window of vulnerability where the
destination user or group can change the target of the operation.
Warn about combining the --dereference, --recursive, and -L flags.

* doc/coreutils.texi (warnOptDerefWithRec): Add macro.
(node chown invocation): Add it to --dereference and -L.
(node chgrp invocation): Likewise.

See also: CVE-2017-18018