docs/ci: rewrite the "farm maintenance ^ other change" rule to mean what we actually...

docs/ci: rewrite the "farm maintenance ^ other change" rule to mean what we actually meant

Sometimes updates to the farm config are needed before re-enabling it,
or updating test expectations with new failures that happened while the
farm was down, etc.

These need to be allowed, as the alternative is to update the
config/expectations/etc. blindly in one MR, merge it, and then merge
another MR with the farm re-enablement.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25560>