core/docs/core/config/namespaces.md: Add @mailbox_defaults

Author: nauski

docs/core/config/namespaces.md

@@ -42,6 +42,30 @@ All visible namespaces must have the same separator.
 
 Inside each namespace there is a list of folders, which form a sub-hierarchy.
 
+::: info
+Enable default INBOX namespace and English language mailbox names with mailbox_special_use flags with [[setting,@mailbox_defaults,english]]
+Using this setting will set these settings by default:
+
+::: details Click to show settings
+```[dovecot.conf]
+    namespace inbox {
+    inbox = yes
+    mailbox Drafts {
+      special_use = "\\Drafts"
+    }
+    mailbox Junk {
+      special_use = "\\Junk"
+    }
+    mailbox Sent {
+      special_use = "\\Sent"
+    }
+    mailbox Trash {
+      special_use = "\\Trash"
+    }
+  }
+```
+:::
+
 ## Folder Names
 
 Each folder has a name. In configuration files and log files Dovecot almost