Added note about methods with trailing underscores to module docs.

Not fantastically impressed with what I've written and welcome for someone else to have a stab at it - trying to rectify that it's not clear from the docs ANYWHERE that, e.g., salt.states.alternatives.set_ must be specified as 'alternatives.set' or 'alternatives: - set' in the sls.  'alternatives.set_' or 'alternatives: - set_' will not work.

As a python coder it's obvious what's going on to avoid the name clash in the source but I wouldn't expect all salt users to be python programmers so it needs mentioning somewhere, imho.