Currently, for multi-build updates,
Package Update Guide gives different instructions
in Rawhide and early branched / Multiple Packages
and Later Branched and stable releases / Updating inter-dependent packages
For Rawhide, side tags are prescribed,
but for stable releases, buildroot overrides.
The community consensus seems to be that side tags are superior.
Thus, side tags should be presented as the first option for both cases.
Buildroot overrides should be documented separately
and the cases where they may still be useful described.

Other pages containing related material:

• Using the Koji build system talks about Chained builds
  and mentions buildroot overrides in that context,
  but says nothing about side tags.

• Rawhide Gating docs have a section about multi-build updates
  which describes the preferred process correctly.

• Discussion on the devel mailing list