Document the limitations of the infer sync with multi-stage Dockerfiles

[briandealwis]

The Docker inferred sync only looks at ADD/COPY commands from the last stage in a multi-stage Dockerfile (#6524). This should be documented, with possible workarounds.

skaffold/pkg/skaffold/docker/syncmap.go

Lines 38 to 39 in d3839f4

	// only the COPY/ADD commands from the last image are syncable
	fts, err := readCopyCmdsFromDockerfile(ctx, true, absDockerfilePath, workspace, buildArgs, cfg)

I believe this decision was made to avoid handling transitive copies in multi-stage Dockerfiles.