[#1294] docs: Fix description of shard switching mode

Signed-off-by: Alexander Chuprov <a.chuprov@yadro.com>

docs/shard-modes.md

@@ -51,10 +51,7 @@ However, all mode changing operations are idempotent.
 
 ## Automatic mode changes
 
-Shard can automatically switch to a `degraded-read-only` mode in 3 cases:
-1. If the metabase was not available or couldn't be opened/initialized during shard startup.
-2. If shard error counter exceeds threshold.
-3. If the metabase couldn't be reopened during SIGHUP handling.
+A shard can automatically switch to `read-only` mode if its error counter exceeds the threshold.
 
 # Detach shard