docs: update help for emqx ctl license command

apps/emqx_license/src/emqx_license.app.src

@@ -1,6 +1,6 @@
 {application, emqx_license, [
     {description, "EMQX License"},
-    {vsn, "5.1.0"},
+    {vsn, "5.1.1"},
     {modules, []},
     {registered, [emqx_license_sup]},
     {applications, [kernel, stdlib, emqx_ctl, emqx_gateway]},

apps/emqx_license/src/emqx_license_cli.erl

@@ -41,7 +41,8 @@ license(_) ->
     emqx_ctl:usage(
         [
             {"license info", "Show license info"},
-            {"license update <License>", "Update license given as a string"}
+            {"license update '<License>'|'file:///tmp/emqx.lic'",
+                "Update license given as a string\nor referenced by a file path via 'file://' prefix"}
         ]
     ).