[Doc] ROCm CI is no longer disabled (#2896)

#2874 re-enabled the ROCm leg on a self-hosted gfx942 (MI300X) runner, so the
"temporarily disabled" note in the backend support table is now stale.

Notes that coverage is gfx942 only. The hardware column already advertises both
gfx942 and gfx950 paths, and the two genuinely diverge (software pipelining,
fp8 e4m3 FNUZ vs OCP encoding, __launch_bounds__), so the gap is worth stating
rather than implying CI covers both.