buildman: Document the members of BuilderJob

This class has a few more members now. Add documentation for them and fix
a nit in the 'commits' comment.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builderthread.py b/tools/buildman/builderthread.py
index 570c1f6..1e52ef8 100644
--- a/tools/buildman/builderthread.py
+++ b/tools/buildman/builderthread.py
@@ -39,11 +39,15 @@
 
     Members:
         board: Board object to build
-        commits: List of commit options to build.
+        commits: List of Commit objects to build
+        keep_outputs: True to save build output files
+        step: 1 to process every commit, n to process every nth commit
     """
     def __init__(self):
         self.board = None
         self.commits = []
+        self.keep_outputs = False
+        self.step = 1
 
 
 class ResultThread(threading.Thread):