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>

kas/plugins/shell.py

@@ -20,8 +20,21 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 """
-    This module contains a kas plugin that opens a shell within the kas
-    environment
+    This plugin implements the ``kas shell`` command.
+
+    When this command is executed, kas will checkout repositories, setup the
+    build environment and then start a shell in the build environment. This
+    can be used to manually run ``bitbake`` with custom command line options
+    or to execute other commands such as ``runqemu``.
+
+    For example, to start a shell in the build environment for the file
+    ``kas-project.yml`` you could run::
+
+        kas shell kas-project.yml
+
+    Or to invoke qemu to test an image which has been built::
+
+        kas shell kas-project.yml -c 'runqemu'
 """
 
 import logging