bfd_perform_relocation and howto special_function bfd params

Describe the output_bfd parameter of bfd_perform_relocation, in
pariticular that it is NULL when final linking.

	* reloc.c (bfd_perform_relocation): Comment on abfd and
	output_bfd parameters in the function description.
	(reloc_howto_struct <special_function>): Refer to above.