Improve pytest_plugins docs

As discussed in #2246

doc/en/writing_plugins.rst

@@ -236,20 +236,31 @@ import ``helper.py`` normally.  The contents of
 Requiring/Loading plugins in a test module or conftest file
 -----------------------------------------------------------
 
-You can require plugins in a test module or a conftest file like this::
+You can require plugins in a test module or a ``conftest.py`` file like this:
 
-    pytest_plugins = "name1", "name2",
+.. code-block:: python
+
+    pytest_plugins = ["name1", "name2"]
 
 When the test module or conftest plugin is loaded the specified plugins
-will be loaded as well.  You can also use dotted path like this::
+will be loaded as well. Any module can be blessed as a plugin, including internal
+application modules:
+
+.. code-block:: python
 
     pytest_plugins = "myapp.testsupport.myplugin"
 
-which will import the specified module as a ``pytest`` plugin.
+``pytest_plugins`` variables are processed recursively, so note that in the example above
+if ``myapp.testsupport.myplugin`` also declares ``pytest_plugins``, the contents
+of the variable will also be loaded as plugins, and so on.
 
-Plugins imported like this will automatically be marked to require
-assertion rewriting using the :func:`pytest.register_assert_rewrite`
-mechanism.  However for this to have any effect the module must not be
+This mechanism makes it easy to share textures within applications or even
+external applications without the need to create external plugins using 
+the ``setuptools``'s entry point technique.
+
+Plugins imported by ``pytest_plugins`` will also automatically be marked
+for assertion rewriting (see :func:`pytest.register_assert_rewrite`). 
+However for this to have any effect the module must not be
 imported already; if it was already imported at the time the
 ``pytest_plugins`` statement is processed, a warning will result and
 assertions inside the plugin will not be re-written.  To fix this you