vty: fix doc str for 'reset sgsn state'

Document all keywords of the 'reset sgsn state' command: set the same doc
string for all three.

Also fixes the build after libosmocore
I1f18e0e41da4772d092d71261b9e489dc1598923, which resulted in HIDDEN commands
coming up in the VTY reference dumping. Note that libosmocore
I92c3c66ff69c186234276c64478d6342e061d25e will again remove this breakage by
omitting hidden commands.

Change-Id: I8b6e8615e409266910f2f76a10ced9ab33e4de91
diff --git a/src/gprs/sgsn_vty.c b/src/gprs/sgsn_vty.c
index 63985bc..87c6884 100644
--- a/src/gprs/sgsn_vty.c
+++ b/src/gprs/sgsn_vty.c
@@ -801,10 +801,14 @@
 	vty_out(vty, "    Use count: %u%s", gsub->use_count, VTY_NEWLINE);
 }
 
+#define RESET_SGSN_STATE_STR \
+      "Remove all known subscribers, MM contexts and flush BSSGP queues." \
+      " Useful only when running tests against the SGSN\n"
+
 DEFUN_HIDDEN(reset_sgsn_state,
       reset_sgsn_state_cmd,
       "reset sgsn state",
-      "Remove all known subscriber, MM ctx and flush BSSGP queues Useful when running tests against the SGSN")
+      RESET_SGSN_STATE_STR RESET_SGSN_STATE_STR RESET_SGSN_STATE_STR)
 {
 	struct gprs_subscr *subscr, *tmp_subscr;
 	struct sgsn_mm_ctx *mm, *tmp_mm;