"Fixes:" tags on commits with Cc: stable

Hi,

regarding a recent fix for AMDGPU which was tagged with Cc: stable, Greg
wrote that the "Fixes:" tag in that commit was wrong from his point of
view: It referenced a commit which was not contained in the mainline
kernel, but only in the drm-next branch (for the next merge window). The
patch from the referenced commit had been cherry-picked for the mainline
kernel via the drm-fixes branch, so Greg argued that the "Fixes:" tag
should have referenced that commit because a reference to a non-mainline
commit doesn't work with the stable team's tooling, which was confirmed
by Sasha.

However, Alex mentioned that some time ago it was determined that the
"Fixes" tag should reference the original commit (drm-next is upstream
for drm-fixes I guess) so that you could follow the cherry-pick -x chain.

As far as I can tell, the documentation doesn't say explicitly what is
correct. Since this issue might be relevant for more people, I'm asking
for a clarification so that the issue can be avoided in the future. Most
likely the documentation should be made clear on this matter.

Regards,
Viktor