Make docstring for hmset less ambiguous

Original docstring:
Sets each key in the ``mapping`` dict to its corresponding value in the hash ``name``

..this sounds like it updates the ``mapping`` dict from the values in the hash ``name``.

Altered docstring:
Set key to value within hash ``name`` for each corresponding key and value from the ``mapping`` dict.

..this sounds like it updates the hash ``name`` from the ``mapping`` dict.