commands: document the "common" class attribute

Switch it to uppercase to make it clear it's a constant, and add
documentation so its usage is clear.

Change-Id: I6d281a66a90b5908b3131585c9945e88cfe815ea
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/309322
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/start.py

@@ -25,7 +25,7 @@ from project import SyncBuffer
 
 
 class Start(Command):
-  common = True
+  COMMON = True
   helpSummary = "Start a new branch for development"
   helpUsage = """
 %prog <newbranchname> [--all | <project>...]