More documentation for the filter by.

[gabelula]

@impronunciable Can you confirm that this is what we want from this endpoint?

@aiboyles Can you check if this is understandable about parameters and response?

[aiboyles]

Could there be more parameters provided in the example call? I'd prefer to have EVERY parameter shown in the example call, so people can see how they're used. For instance:

GET
https://localhost:8080/api/form_submissions/123?skip=y&limit=5&orderby=date&filterby=^(?!test_the_flag)

I also think, looking at the amount of parameters for this and potentially the amount of parameters for other endpoints, that it might make sense to put this into a table where we can list 1) Whether or not the parameter is required, and 2) Possible values for the parameter.

Something like this:

| Parameters  | Required? | Value            | Description     |
|:------------|:----------|:----------------|:----------------|
| form_id     |Y          |Number         |Form id for which we want to see the submissions     |
| skip        |Y          |Number         |Skip the first _n_ submissions      |
| limit       |N          |Number         |Limit to _n_ submission returns      |
| orderby     |N          |`asc` or `dsc` |Order by ascending date (`asc`) or descending date (`dsc`)   |
| filterby    |N          | Regex string (example: `^(?!test_the_flag)`) |Filter by a specific flag using regular expressions. If you want to match all the submissions that do not have a specific flag, use `^(?!test_the_flag)`  |

[aiboyles]

lgtm. I'll make the formatting changes on my end.