A big spring clean of the docstrings (sure long time coming!)

 *  Fix typos,

 *  rewrite clumsy explanations,

 *  omit unnecessary and misleading parts,

 *  remove awkward and "smart" language,

 *  use consistent here-doc formatting across small and big docstrings
    (isolate quote mark triplets on own line.)

Not sure why but 2to3 suggested to enclose the % expression into
parentheses before passing it to print(); I did about the same thing
manually but a bit more readable.