common.mk: document some variables to prevent merge-conflicts

Both the DOCKER_COMPOSE_REF and DOCKER_BUILDX_REF versions are frequently
updated, which made it more likely to run into merge conflicts when
pull requests are merged to update either.

Add a comment in between to reduce the probability for such PRs to
conflict.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

common.mk

@@ -38,7 +38,11 @@ DOCKER_BUILDX_REPO  ?= https://github.com/docker/buildx.git
 REF                ?= HEAD
 DOCKER_CLI_REF     ?= $(REF)
 DOCKER_ENGINE_REF  ?= $(REF)
+# DOCKER_COMPOSE_REF is the version of compose to package. It usually is a tag,
+# but can be a valid git reference in DOCKER_COMPOSE_REPO.
 DOCKER_COMPOSE_REF ?= v2.24.7
+# DOCKER_BUILDX_REF is the version of compose to package. It usually is a tag,
+# but can be a valid git reference in DOCKER_BUILDX_REPO.
 DOCKER_BUILDX_REF  ?= v0.13.0
 
 # Use "stage" to install dependencies from download-stage.docker.com during the