Add comment for local-lxd track

[javierdelapuente]

Applicable spec:

Overview

Rationale

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied.
• The contributing guide was applied.
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs.
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex).
• The changelog is updated with changes that affects the users of the charm.
• The changes do not introduce any regression in code or tests related to LXD runner mode.