update decommission docs

docs/distributed/DECOMMISSION.md

@@ -1,5 +1,12 @@
 ## Decommissioning
 
+Decommissiong is a mechanism in MinIO to drain older pools (usually with old hardware) and migrate the content from such pools to a newer pools (usually better hardware). Decommissioning spreads the data across all pools - for example if you decommission `pool1`, all the data from `pool1` shall be spread across `pool2` and `pool3` respectively.
+
+### Features
+
+- A pool in decommission still allows READ access to all its contents, newer WRITEs will be automatically scheduled to only new pools.
+- All versioned buckets maintain the same order for "versions" for each objects after being decommissioned to the newer pools.
+
 ### How to decommission a pool?
 ```
 λ mc admin decommission start alias/ http://minio{1...2}/data{1...4}
@@ -31,8 +38,7 @@ ERROR: This pool is not scheduled for decommissioning currently.
 ```
 
 ### Canceling a decommission?
-Stop an on-going decommission in progress, mainly used in situations when the load may be
-too high and you may want to schedule the decommission at a later point in time.
+Stop an on-going decommission in progress, mainly used in situations when the load may be too high and you may want to schedule the decommission at a later point in time.
 
 `mc admin decommission cancel` without an argument, lists out any on-going decommission in progress.
 
@@ -44,9 +50,7 @@ too high and you may want to schedule the decommission at a later point in time.
 └─────┴─────────────────────────────────┴──────────────────────────────────┴──────────┘
 ```
 
-> NOTE: Canceled decommission will not make the pool active again, since we might have
-> Potentially partial duplicate content on the other pools, to avoid this scenario be
-> absolutely sure to start decommissioning as a planned activity.
+> NOTE: Canceled decommission will not make the pool active again, since we might have  potentially partial namespace on the other pools, to avoid this scenario be absolutely sure to make decommissioning a planned well thought activity. This is not to be run on a daily basis.
 
 ```
 λ mc admin decommission cancel alias/ http://minio{1...2}/data{1...4}
@@ -71,3 +75,11 @@ If for some reason decommission fails in between, the `status` will indicate dec
 ```
 λ mc admin decommission start alias/ http://minio{1...2}/data{1...4}
 ```
+
+### NOTE
+- Empty delete marker's i.e objects with no other successor versions are not transitioned to the new pool, to avoid any empty metadata being recreated on the newer pool. We do not think this is needed, please open a GitHub issue if you think otherwise.
+
+### TODO
+- Richer progress UI is not present at the moment, this will be addressed in subsequent releases. Currently however a RATE of data transfer and usage increase is displayed via `mc`.
+
+- Transitioned Hot Tier's as pooled setups are not currently supported, attempting to decommission buckets with ILM Transition will be rejected by the server. We will be working on adding support for this in future.