[Doc] Add a link to the V2 Checkpoint specification in the DROP TABLE…

… Feature doc

Support for dropping V2 Checkpoints was added in Delta 3.1. This PR makes sure that the V2 Checkpoint spec is linked in the doc about DROP Table Feature.

docs/source/delta-drop-feature.md

@@ -25,7 +25,7 @@ To remove a Delta table feature, you run an `ALTER TABLE <table-name> DROP FEATU
 You can drop the following Delta table features:
 
 - `deletionVectors`. See [_](delta-deletion-vectors.md).
-- `v2Checkpoint`
+- `v2Checkpoint`. See [V2 Checkpoint Spec](https://github.com/delta-io/delta/blob/master/PROTOCOL.md#v2-spec). Drop support for V2 Checkpoints is available in <Delta> 3.1.0 and above.
 
 You cannot drop other [Delta table features](https://github.com/delta-io/delta/blob/master/PROTOCOL.md#valid-feature-names-in-table-features).