Document RGBLINK's support for SDAS object files

[RGBDS 0.4.1](https://github.com/gbdev/rgbds/releases/tag/v0.4.1) added "GBDK support" (aka RGBLINK support for objects output by SDAS) in July 2020, but we've neglected to document it. Update rgblink.1 to not only mention "RGB objects".

Note that #1008 mentions other bullet points for ostensibly complete "C toolchain" usability. I don't know how essential/blocking any of those are to actually using RGBASM+RGBLINK as part of a C+asm multilanguage project.

Start with rgblink.1 and compare its current wording with the RGBDS 0.4.1 release note about GBDK/SDAS object support. Review #1008 for the other proposed C toolchain points, then update the man page so RGBLINK's SDAS object support is accurately documented; the change is done when the relevant documentation no longer mentions only RGB objects.

Written by the indexing model from the issue text.