Polish the deprecation message in "mix hex.docs" (#294)

lib/mix/tasks/hex/docs.ex

@@ -32,11 +32,15 @@ defmodule Mix.Tasks.Hex.Docs do
 
     case args do
       [] ->
-        deprecation_msg = """
-        [deprecation] Calling mix hex.docs without a command is deprecated, please use:
-          mix hex.publish docs
+        Mix.raise """
+        [deprecation] The "mix hex.docs" command has changed. To use the old
+        behaviour (publishing docs), use:
+
+            mix hex.publish docs
+
+        The new "mix hex.docs" command has to be invoked with at least one
+        argument. Call "mix help hex.docs" for more information.
         """
-        Mix.raise deprecation_msg
       ["fetch" | remaining] ->
         fetch_docs(remaining, opts)
       ["open" | remaining] ->

test/mix/tasks/hex/docs_test.exs

@@ -65,13 +65,8 @@ defmodule Mix.Tasks.Hex.DocsTest do
   end
 
   test "invalid arguments for docs task" do
-    deprecation_msg = """
-    [deprecation] Calling mix hex.docs without a command is deprecated, please use:
-      mix hex.publish docs
-    """
-    assert_raise Mix.Error, deprecation_msg, fn ->
-      Mix.Tasks.Hex.Docs.run([])
-    end
+    exception = assert_raise Mix.Error, fn -> Mix.Tasks.Hex.Docs.run([]) end
+    assert Exception.message(exception) =~ ~s([deprecation] The "mix hex.docs" command has changed)
 
     invalid_args_msg = """
     invalid arguments, expected one of: