doc only: fold map and task descriptions of fn args, markdown for code examples

[nurturenature]

Hi,

I'm starting a new checker for Elle, and in using the documentation for history/fold/task found two places with inaccuracies:

• fold map description in jepsen.history.fold
:reducer* and :combiner* functions don't take a history as the first arg

• jepsen.history/task macro description
  macro doesn't take the single-arity argument vector mentioned

Also added markdown for code examples so the documentation, IDE help, etc is more usable/accessible.

---

A few observations:

The only thing that felt uncertain was the boundary with ec/combine.
If multiple graphs want to share task dependencies, they must be in an analyzer fn that combines the graph/explainers
before use by (ec/combine my-combined-graphs process-graph).
I kept finding myself wanting to pass a dependency tree of tasks vs a function to ec/combine.

And it seems like submitting a task for each :process in a history to concurrently reduce (when you need to reduce over a :process's ops) would be appropriate:

(->> processes
       (map #(h/task history (str :ww-wfr %) [] (ww-wfr-order history % ...)))
       (map deref)
       (reduce g/named-graph-union (g/linear (g/named-graph :ww-wfr (g/digraph))))
       g/forked)

Thanks for such an ergonomic way to process histories so efficiently!