man: document the use of `!' in the importer

Document the shell invoke in the importer for 18.7 and the people
who will want the installer to run early.  This is really not what
we want to do ever since we moved to a `installer' user and try to
enforce a live environment to help people test the look and feel,
either by first impression for new users or for previewing the new
version prior to installation.

src/man/man8/opnsense-importer.8

@@ -40,6 +40,12 @@ utility can import a previous configuration from multiple types of
 disk formats.
 If successful, the old configuration will be replaced.
 .Pp
+The disk selection allows the user to escape to a shell by issuing
+.Sq \&!
+instead.
+This shell can be used to e.g. start an early install process using
+.Xr opnsense-installer 8 .
+.Pp
 The options are as follows:
 .Bl -tag -width ".Fl b" -offset indent
 .It Fl b