Add comment about Thanos, external labels and downsampling. (grafana#…

…2122) (grafana#2362)

* Add comment about Thanos, external labels and downsampling.

Signed-off-by: Peter Štibraný <pstibrany@gmail.com>

* Explain how Mimir uses external labels from Thanos.
Removed section on metaconvert.

Signed-off-by: Peter Štibraný <pstibrany@gmail.com>

* Apply Nick's suggestions.

Signed-off-by: Peter Štibraný <pstibrany@gmail.com>

* Apply suggestions from review.

Signed-off-by: Peter Štibraný <pstibrany@gmail.com>

* Be explicit about what user should do in different scenarios.

Signed-off-by: Peter Štibraný <pstibrany@gmail.com>

* make doc

* Apply review feedback.

Signed-off-by: Peter Štibraný <pstibrany@gmail.com>

* Don't mention label used by compactor at all.

Signed-off-by: Peter Štibraný <pstibrany@gmail.com>

* Add notes about rewriting blocks and offline deduplication.

Signed-off-by: Peter Štibraný <pstibrany@gmail.com>
(cherry picked from commit 78f18f5)