Fix error in gmountver(8) man page

Also fixes the usage statement for the binary to use the correct terms (provider and geom name, rather than dev and prov, which is incorrect in the latter case)
Sync the man page summary with the new usage statement

PR:		199540
Differential Revision:	https://reviews.freebsd.org/D2329
Submitted by:	Fabian Keil
Reviewed by:	trasz
Approved by:	eadler (mentor)
Obtained from:	ElectroBSD (original)
MFC after:	1 week
Sponsored by:	ScaleEngine Inc.

sbin/geom/class/mountver/geom_mountver.c

@@ -43,14 +43,14 @@ struct g_command class_commands[] = {
 	    {
 		G_OPT_SENTINEL
 	    },
-	    "[-v] dev ..."
+	    "[-v] prov ..."
 	},
 	{ "destroy", G_FLAG_VERBOSE, NULL,
 	    {
 		{ 'f', "force", NULL, G_TYPE_BOOL },
 		G_OPT_SENTINEL
 	    },
-	    "[-fv] prov ..."
+	    "[-fv] name"
 	},
 	G_CMD_SENTINEL
 };

sbin/geom/class/mountver/gmountver.8

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 27, 2015
+.Dd May 18, 2015
 .Dt GMOUNTVER 8
 .Os
 .Sh NAME
@@ -35,11 +35,11 @@
 .Nm
 .Cm create
 .Op Fl v
-.Ar dev ...
+.Ar prov ...
 .Nm
 .Cm destroy
 .Op Fl fv
-.Ar prov ...
+.Ar name
 .Nm
 .Cm list
 .Nm
@@ -66,8 +66,11 @@ The first argument to
 indicates an action to be performed:
 .Bl -tag -width ".Cm destroy"
 .It Cm create
-Cache the given devices with specified
-.Ar name .
+Enable mount verification for the given provider.
+If the operation succeeds, a new GEOM provider will be created using the
+given provider's name with a
+.Ql .mountver
+suffix.
 The kernel module
 .Pa geom_mountver.ko
 will be loaded if it is not loaded already.