LD/doc: Clarify `-rpath' option's semantics WRT link-time dependencies

Mention in the description of the `-rpath' LD option the restrictions
the option has with respect to searching link-time dependencies of
shared objects referred in the link.  Previously these restrictions were
only documented along with the `-rpath-link' option, which may not be
the place one would consider when looking for the semantics of `-rpath'.

Copy the relevant part of the `-rpath-link' option description then,
splitting the now overlong paragraph into three, for legibility.

	ld/
	* ld.texi (Options): Also document `-rpath' option restrictions
	in the description of the option itself.