Documentation of the fields in the problem types

[ChrisRackauckas]

#151 added a bunch of documentation for the problem types, but took some shortcuts when doing the fields. The only one that did it properly was the ODEProblem:

SciMLBase.jl/src/problems/ode_problems.jl

Lines 69 to 82 in 80d0756

	struct ODEProblem{uType,tType,isinplace,P,F,K,PT} <:
	AbstractODEProblem{uType,tType,isinplace}
	"""The ODE is `du = f(u,p,t)` for out-of-place and f(du,u,p,t) for in-place."""
	f::F
	"""The initial condition is `u(tspan[1]) = u0`."""
	u0::uType
	"""The solution `u(t)` will be computed for `tspan[1] ≤ t ≤ tspan[2]`."""
	tspan::tType
	"""Constant parameters to be supplied as the second argument of `f`."""
	p::P
	"""A callback to be applied to every solver which uses the problem."""
	kwargs::K
	problem_type::PT
	"""An internal argument for storing traits about the solving process."""

All of the others didn't do it via docstrings on the fields. We should get that fixed, it's a nice first issue since it's quite cookie-cutter fix!