Update description of the -plugin option used by the linker, ar and nm.

Update description of the -plugin option used by the linker, ar and nm.

	PR 20343
ld	* ld.texinfo (Options): Extend documentation of the --plugin
	option.  Include a description of where the plugins should be
	located.

binutils* doc/binutils.texi (ar): Extend documentation of the --plugin
	option.  Include a description of where the plugins should be
	located.
	(nm): Likewise.