docs: update undefined var check reference

[dvdksn]

Updates the checks reference for the UndefinedVar rule to clarify the following:

• Variables declared in base images are considered valid (no warn)
• This rule does not apply to shell form instructions

Related issues

• UndefinedVar build check behaviour for PATH-like variables docker/buildx#2751

[crazy-max]

Looks like a legitimate ci failure: https://github.com/moby/buildkit/actions/runs/11496293740/job/31997650750?pr=5449#step:5:391

------
 > [validate 1/1] RUN --mount=target=/context   --mount=target=.,type=tmpfs <<EOT (set -e...):
0.773 ERROR: Dockerfile docs result differs. Please update with "make docs-dockerfile"
0.795  M frontend/dockerfile/docs/rules/undefined-var.md