plugins: Improve documentation of existing modules

The docstrings for the build and shell plugins are extended so that they
clearly describe each plugin and give brief examples of their use.

Also fix a typo in the build command help message.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/userguide.rst

@@ -65,6 +65,23 @@ Use Cases
     $ kas build kas-project.yml
 
 
+Plugins
+-------
+
+kas sub-commands are implemented by a series of plugins. Each plugin
+typically provides a single command.
+
+``build`` plugin
+~~~~~~~~~~~~~~~~
+
+.. automodule:: kas.plugins.build
+
+``shell`` plugin
+~~~~~~~~~~~~~~~~
+
+.. automodule:: kas.plugins.shell
+
+
 Project Configuration
 ---------------------