RGBDS 0.4.1 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.