example-config: Update mail_temp_dir comment

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

@@ -165,7 +165,10 @@ namespace inbox {
 # methods. NFS users: flock doesn't work, remember to change mmap_disable.
 #lock_method = fcntl
 
-# Directory in which LDA/LMTP temporarily stores incoming mails >128 kB.
+# Directory where mails can be temporarily stored. Usually it's used only for
+# mails larger than >= 128 kB. It's used by various parts of Dovecot, for
+# example LDA/LMTP while delivering large mails or zlib plugin for keeping
+# uncompressed mails.
 #mail_temp_dir = /tmp
 
 # Valid UID range for users, defaults to 500 and above. This is mostly