docs: update authz docs with pruning details (#14957)

(cherry picked from commit 21378e3)

x/authz/README.md

@@ -116,6 +116,12 @@ https://github.com/cosmos/cosmos-sdk/blob/v0.47.0-rc1/proto/cosmos/authz/v1beta1
 
 We are maintaining a queue for authz pruning. Whenever a grant is created, an item will be added to `GrantQueue` with a key of expiration, granter, grantee.
 
+In `EndBlock` (which runs for every block) we continuously check and prune the expired grants by forming a prefix key with current blocktime that passed the stored expiration in `GrantQueue`, we iterate through all the matched records from `GrantQueue` and delete them from the `GrantQueue` & `Grant`s store.
+
+```go reference
+https://github.com/cosmos/cosmos-sdk/blob/5f4ddc6f80f9707320eec42182184207fff3833a/x/authz/keeper/keeper.go#L378-L403
+```
+
 * GrantQueue: `0x02 | expiration_bytes | granter_address_len (1 byte) | granter_address_bytes | grantee_address_len (1 byte) | grantee_address_bytes -> ProtocalBuffer(GrantQueueItem)`
 
 The `expiration_bytes` are the expiration date in UTC with the format `"2006-01-02T15:04:05.000000000"`.