Doc review: update the Listing Plugins section

The output of the plugins command has changed. It now includes plugins
that may fail to load or even have been disabled (usually by themselves).

Let's make the user aware of these new possibilities and update the UI
examples.

Changes from v1:
 * Fix wording suggestion of "cleaner" instead of "clean"
Signed-off-by: NCleber Rosa <crosa@redhat.com>

docs/source/Plugins.rst

 .. _Writing Plugins:
 
-Avocado Plugins
-===============
+Plugin System
+=============
+
+Avocado has a plugin system that can be used to extended it in a clean way.
+
+Listing plugins
+---------------
+
+The ``avocado`` command line tool has a builtin ``plugins`` command that lets
+you list available plugins. The usage is pretty simple::
+
+ $ avocado plugins
+ Plugins enabled:
+    config                               Implements the avocado 'config' subcommand
+    distro                               Implements the avocado 'distro' subcommand
+    exec_path                            Implements the avocado 'exec-path' subcommand
+    gdb                                  Run tests with GDB goodies enabled
+    htmlresult                           HTML job report
+    json                                 JSON output
+    ...
+
+Since plugins are (usually small) bundles of Python code, they may fail to load if
+the Python code is broken for any reason. Example::
+
+ $ avocado plugins
+ Plugins enabled:
+    config                               Implements the avocado 'config' subcommand
+  ...
+  Unloadable plugin modules:
+    avocado.core.plugins.htmlresult      ImportError No module named pystache
+
+Besides load errors, plugins may also disable themselves due to, say, missing
+requirements on your environment. This is the case for the ``run_vm`` plugin when
+it's run on machine not capable of (``libvirt`` based) virtualization::
+
+ $ avocado plugins
+  Plugins enabled:
+    config                               Implements the avocado 'config' subcommand
+  ...
+  Plugins disabled:
+    run_vm                               Disabled during plugin configuration
 
-In order to keep the code base approachable, and allow for cleanly extending
-the test runner functionality, Avocado has a plugin system. In order to
-exemplify the concepts involved, let's use another old time favorite,
-the "Print hello world" theme.
-
-List available plugins
-----------------------
-
-Avocado has a builtin ``plugins`` command that lets you list available
-plugins::
-
-    $ avocado plugins
-    Plugins loaded:
-        test_lister - Implements the avocado 'list' functionality. (Enabled)
-        sysinfo - Collect system information and log. (Enabled)
-        test_runner - Implements the avocado 'run' functionality. (Enabled)
-        xunit - xUnit output plugin. (Enabled)
-        plugins_list - Implements the avocado 'plugins' functionality. (Enabled)
-
-Now, let's write a simple plugin.
 
 Writing a plugin to extend the runner functionality
 ---------------------------------------------------