example-config: Add mail_sort_max_read_count setting.

doc/example-config/conf.d/10-mail.conf

@@ -248,6 +248,11 @@ namespace inbox {
 # These should exist only after Dovecot dies in the middle of saving mails.
 #mail_temp_scan_interval = 1w
 
+# How many slow mail accesses sorting can perform before it returns failure.
+# With IMAP the reply is: NO [LIMIT] Requested sort would have taken too long.
+# The untagged SORT reply is still returned, but it's likely not correct.
+#mail_sort_max_read_count = 0
+
 protocol !indexer-worker {
   # If folder vsize calculation requires opening more than this many mails from
   # disk (i.e. mail sizes aren't in cache already), return failure and finish