As a user and/or extension developer, it can be a source of confusion when,
on performing a `sphinx-build` for a cached project,
an unchanged configuration variable is always marked as changed (triggering a full rebuild).
This is often due to the variable being an unpicklable value,
which is silently omitted from the cache (i.e. the `environment.pickle`).
This commit introduces a specific warning for when a configuration variable is omitted from the cache,
allowing users to understand the root of the problem.
430866e029