clarify documentation about fetchData function

[elray1]

• Function arguments are described in the readme under a section titled App.initialize() args; it could be helpful to repeat them again below in the fetchData data format section
• In that section, we could add language explaining that for target data, the expected behavior is to return a version of the target data that was available as of the specified referenceDate

[zkamvar]

It would also be good to reword it to say "target time series data" instead of "truth data".

[zkamvar]

Your suggestions would definitely help.

I think one of the biggest problems that I encountered was that I didn't have any clear examples beyond the example in predtimechart/src/index.html. Without a narrative behind this example, I could only see that the updated target data was simply adding dates, but nothing else changed:

https://github.com/reichlab/predtimechart/blob/6678c66fe7a3faab2bcc5186733e7a25cf504d44/src/index.html#L107-L117

Since the target data in the flusight hub was a single file, I had assumed that we were just appending new data and not amending existing data (which in hindsight was a foolish assumption, because I even designed a figure that shows that scenario) and just went with it.

Most of the documentation that exists is low-level reference documentation, but for someone without much context to go from, it's not clear where to start or what features are important or auxiliary. A more explanatory form of documentation would have been helpful in this situation so that it was clear where the data were coming from and what the context was for the different modes of visualization.