feature #4657 Update assetic watch command (xtreamwayz)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4657).

Discussion
----------

Update assetic watch command

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6
| Fixed tickets | -

I'm getting this message:

    The --watch option is deprecated. Please use the assetic:watch command instead.

Commits
-------

1009ed4 Typo
987a31a Improve assetic:watch text
8c38ec8 Update asset_management.rst

cookbook/assetic/asset_management.rst

@@ -540,12 +540,16 @@ need to dump them manually. To do so, run the following:
 
 This physically writes all of the asset files you need for your ``dev``
 environment. The big disadvantage is that you need to run this each time
-you update an asset. Fortunately, by passing the ``--watch`` option, the
-command will automatically regenerate assets *as they change*:
+you update an asset. Fortunately, by using the ``assetic:watch`` command,
+assets will be regenerated automatically *as they change*:
 
 .. code-block:: bash
 
-    $ php app/console assetic:dump --watch
+    $ php app/console assetic:watch
+
+The ``assetic:watch`` command was introduced in AsseticBundle 2.4. In prior 
+versions, you had to use the ``--watch`` option of the ``assetic:dump`` 
+command for the same behavior.
 
 Since running this command in the ``dev`` environment may generate a bunch
 of files, it's usually a good idea to point your generated asset files to