NOTES.UNIX: expand the description of RPATHs

Hopefully, this will make it more clear that it isn't only ELF
specific, even though there is a part that is (or even more
restrictively GNU ld.so specific).

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6520)