minor #1399 Added argument description to twig asset() method (DaveC49)

This PR was squashed before being merged into the 2.2.x-dev branch (closes #1399).

Discussion
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Added argument description to twig asset() method

I have added a description of the arguments for the asset() method. I didn't find them obvious when I first used the AssetServiceProvider, although it makes sense in hindsight. The description would have saved me some time getting the paths right

Commits
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2f5d870f Added argument description to twig asset() method

doc/providers/asset.rst

@@ -55,7 +55,10 @@ Registering
 Usage
 -----
 
-The AssetServiceProvider is mostly useful with the Twig provider:
+The AssetServiceProvider is mostly useful with the Twig provider using the asset(method).
+This has two string arguments. In the case of named packages, the first is the path relative to
+the base_path specified in the package definition and the second is the package name. For unmamed
+packages, there is only one argument, the path relative to the assets folder:
 
 .. code-block:: jinja
 
@@ -65,5 +68,7 @@ The AssetServiceProvider is mostly useful with the Twig provider:
 
     {{ asset_version('/css/foo.png') }}
     
+
+
 For more information, check out the `Asset Component documentation
 <https://symfony.com/doc/current/components/asset/introduction.html>`_.