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

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

docs/shard-modes.md

@@ -51,11 +51,13 @@ However, all mode changing operations are idempotent.
 
 ## Automatic mode changes
 
-Shard can automatically switch to a `degraded-read-only` mode in 3 cases:
+Shard can automatically switch to a `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.
 
+If switching to `read-only` is impossible, the shard will switch to `degraded-read-only`.
+
 # Detach shard
 
 To detach a shard use `frostfs-cli control shards detach` command. This command removes the shards from the storage