Documentation for `arx_args_list(adjust_latency)` is incomplete

[brookslogan]

• "extend_lags" only mentions lags, but also appears to have the effects of "adjust_ahead" as well
• (I'm not sure if "extend_ahead" is quite the best name... if both this and the lag part of "adjust_lags" are changing to index relative to the forecast_date/as_of, then they can't both be "extending".)
• arx_args_list() also has an error message you can get mentioning something like a "locf" possibility, but it's not documented. Did we deprecate this?

[brookslogan]

Also, it seems like one of "extend_ahead" / "adjust_lags" is probably aligning with the wrong lags in training data, and the one that's not may or may not have issues if the max time_value isn't the same for every epikey (from #409 I'm guessing maybe it is a problem?). In docs we should note these properties. And tell users when to use what thing.