2c58471 Fix and reword some docstrings again

Authored and Committed by cqi 5 years ago
    Fix and reword some docstrings again
    
    During I'm reading through the code base to learn MBS, I found out some
    typos and minor issues in some other docstrings, and I also found some
    docstrings with extra informative description could make it easier to
    understand the code.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>