in "mox import ..." help output, make it more clear what should be done to make mbox/maildir archives accessible to the mox process

for issue #79 reported by mattfbacon, thanks!

import.go

@@ -27,7 +27,13 @@ import (
 
 // todo: add option to trust imported messages, causing us to look at Authentication-Results and Received-SPF headers and add eg verified spf/dkim/dmarc domains to our store, to jumpstart reputation.
 
-const importCommonHelp = `By default, messages will train the junk filter based on their flags and, if
+const importCommonHelp = `The mbox/maildir archive is accessed and imported by the running mox process, so
+it must have access to the archive files. The default suggested systemd service
+file isolates mox from most of the file system, with only the "data/" directory
+accessible, so you may want to put the mbox/maildir archive files in a
+directory like "data/import/" to make it available to mox.
+
+By default, messages will train the junk filter based on their flags and, if
 "automatic junk flags" configuration is set, based on mailbox naming.
 
 If the destination mailbox is the Sent mailbox, the recipients of the messages
@@ -45,9 +51,6 @@ func cmdImportMaildir(c *cmd) {
 ` + importCommonHelp + `
 Mailbox flags, like "seen", "answered", will be imported. An optional
 dovecot-keywords file can specify additional flags, like Forwarded/Junk/NotJunk.
-
-The maildir files/directories are read by the mox process, so make sure it has
-access to the maildir directories/files.
 `
 	args := c.Parse()
 	if len(args) != 3 {
@@ -63,11 +66,7 @@ func cmdImportMbox(c *cmd) {
 
 Using mbox is not recommended, maildir is a better defined format.
 
-` + importCommonHelp + `
-
-The mailbox is read by the mox process, so make sure it has access to the
-maildir directories/files.
-`
+` + importCommonHelp
 	args := c.Parse()
 	if len(args) != 3 {
 		c.Usage()