Fixed the default value of the $prompt variable, document what

happens if $prompt is unset.

sys/boot/common/help.common

@@ -249,7 +249,7 @@
 	Variable substitution is performed on the prompt.  The default 
 	prompt can be set with:
 
-		set prompt=\$currdev>
+		set prompt=\${interpret}
 
 ################################################################################
 # Tset Srootdev DSet the root filesystem

sys/boot/common/loader.8

@@ -395,7 +395,11 @@ Value of
 .Nm Ns 's
 prompt.
 Defaults to
-.Dq Li "${currdev}>" .
+.Dq Li "${interpret}" .
+If variable
+.Va prompt
+is unset, the default prompt is
+.Ql > .
 .It Va root_disk_unit
 If the code which detects the disk unit number for the root disk is
 confused, e.g.\& by a mix of SCSI and IDE disks, or IDE disks with