Include example of partial stage reloads in docs

[WarmCyan]

Curifactory has no way internally of dealing with partial stage cache hits (e.g. one thing in cache for a stage is found but not another)

A way to deal with this manually is through record.stage_cachers:

@cf.stage(None, ["thing1", "thing2"], [PickleCacher]*2)
def make_things(record):
    # in the case where thing1 is in cache but thing2 is not, the whole stage will re-run
    # if we know this is a possibility, and maybe thing1 takes a very long time to compute, we could
    # manually handle this inside the stage (assuming it is not cleaner to separate out thing1 and thing2 production
    # into separate stages)
    if record.stage_cachers[0].check():
        thing1 = record.stage_cachers[0].load()
    else:
        # create thing1 the normal way
        ...
    # create thing2
    ...
    return thing1, thing2