doveadm: Fill out example information for commands

Also add missing text descriptions.

Author: slusarz

.github/actions/spelling/expect.txt

@@ -562,6 +562,7 @@ offenburg
 OIDC
 olc
 oldkey
+oldpassword
 oldpath
 oldstats
 onelevel
@@ -798,6 +799,7 @@ terabyte
 testdomain
 testmbox
 testpass
+testprefix
 testshared
 textcat
 thead