Add module level docstring explaining when _pmf functions are useful.
Add module level docstring explaining when _pmf functions are useful.

chebee7i/dit,Autoplectic/dit,Autoplectic/dit,dit/dit,Autoplectic/dit,dit/dit,chebee7i/dit,dit/dit,chebee7i/dit,chebee7i/dit,dit/dit,Autoplectic/dit,dit/dit,Autoplectic/dit