why link when have ln

[Dan Jacobson]

At least on the link and unlink Info pages, and probably also on their
man pages, say why one would need them instead of ln and rm.

Also saying
   That makes it a little harder to remove files named `--help' and
   `--version', so when the environment variable `POSIXLY_CORRECT' is set,
is a tease, unless you mention using ./--help etc.