[NFC][docs][HIP] Update HIP docs around hipstdpar and SPIR-V (#124803)

This addresses an odd ommision from the 19 release cycle, wherein we
upstreamed HIPSTDPAR support without adding the relevant documentation.
As an added bonus, we also remove a reference to `amdgcnspirv` not
mixing with concrete targets, as that limitation has been addressed.

Author: AlexVlx