rotate: narrow a pair credential to one consuming module (hq issue 268)

A machine runs many consumers of one provision, each with its own
credential. When one module leaks its credential, `rotate <provision>
--consumer <machine>` was the narrowest act and replaced every module's
on that machine, restarting all of them. --module (and the verb's
module argument beside provision) rotates only that module's.

internal/catalogue/verbs.go

@@ -142,7 +142,7 @@ var ControllerVerbs = []Verb{
 			"node":   "the machine's name; without it, every machine that is behind",
 			"behind": "\"true\": every machine that is behind, the whole mesh — the same as naming none, said outright; not with node",
 		}, nil, "behind")},
-	{Name: "rotate", Description: "Replace a credential. A pair credential, by provision (and a consuming machine, " +
+	{Name: "rotate", Description: "Replace a credential. A pair credential, by provision (and a consuming machine and module, " +
 		"else every holder): both ends are re-sent together. Or a module's own secret, by machine, module and " +
 		"name: made anew and the machine sent, so the module starts again on it — only for a secret its " +
 		"definition says it reads at start; a value given to the mesh, or one the module applies to a backend, is refused with the reason.",
@@ -150,7 +150,7 @@ var ControllerVerbs = []Verb{
 			"provision": "a pair credential: the provision whose credential to replace",
 			"consumer":  "with provision: only the holder on this machine (optional)",
 			"node":      "an own secret: the machine",
-			"module":    "an own secret: the module",
+			"module":    "an own secret: the module; with provision: only this consuming module's credential (optional)",
 			"secret":    "an own secret: its name in the module's definition",
 		}, nil)},
 	{Name: "issue", Description: "Give a module on a machine its account on the bus: minted, and sealed to the " +