Make role of filterBy clearer in docs

[wetneb]

What issue does this pull request resolve?
The description of the filterBy callback is not clear enough.

What changes did you make?
I added a description of the arguments and return value expected from this callback.

Is there anything that requires more attention while reviewing?
I don't think so.

[ericgio]

Hi @wetneb, thanks for the pull request. The docs have a dedicated section on filtering with a more in-depth description of the API. The props table is meant to be more of an overview of the available props.

[wetneb]

I am not sure what would be the harm in adding that information there too? Would you prefer it to be a direct link to https://github.com/ericgio/react-bootstrap-typeahead/blob/master/docs/Filtering.md#filterby maybe?

Overall my point is it is a bit frustrating for the user to read about "a callback" without knowing what its signature is. I think it is reasonable to expect that a list of props also documents the types of each of these props.

[ericgio]

Yes, I think a direct link would help surface the in-depth docs. Adding the actual signature could be a more concise way of presenting the information directly (eg: (option: Object|String, props: Object) => boolean), but it doesn't look like that works very well within table markdown.

[ericgio]

This is great, thank you.