Update docs: How Turing implements AbstractMCMC

[BenjaminJCox]

The documentation makes numerous references to functions that are not used by the current version of AbstractMCMC, and contains an outdated extension of assume (potentially among other things). It seems as though this tutorial has not been kept up to date, and I am unsure as to whether it is correct anymore.

I have noticed that:

• ISState is defined here despite not being present in is.jl
• No mention is made of DynamicPPL.initialstep, DynamicPPL.initialsampler
• DynamicPPL.assume and DynamicPPL.observe are extended differently in the docs than in is.jl
• lines 27-29 of is.jl are devoid of explanation in the docs

This is confusing as someone looking to contribute, but I anticipate that someone more familiar with the relevant code would easily be able to correct this.