wic: Add missing docstrings to plugin classes

Docstings from plugin classes are used as part of
output of 'wic help plugins'. Adding them makes help page
more informative.

(From OE-Core rev: d4414b45c81ab539f75a7bb16fc6412c30dfc45f)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/plugins/imager/direct_plugin.py

@@ -33,6 +33,15 @@ import wic.imager.direct as direct
 from wic.pluginbase import ImagerPlugin
 
 class DirectPlugin(ImagerPlugin):
+    """
+    Install a system into a file containing a partitioned disk image.
+
+    An image file is formatted with a partition table, each partition
+    created from a rootfs or other OpenEmbedded build artifact and dd'ed
+    into the virtual disk. The disk image can subsequently be dd'ed onto
+    media and used on actual hardware.
+    """
+
     name = 'direct'
 
     @classmethod