htools: further docstring fixes

This adds parameter documentation for Cluster.iMoveToJob (I think it
was not clear if the new or old node list is needed) and fixes other
docstring style issues.

After this patch, all modules except for CLI.hs (which has many
obvious declarations for command-line options) and QC.hs (unittests)
have 100% doc-strings.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>