Add more documentation to public structs and functions

[NickLarsenNZ]

The docs are a little bare

• https://godoc.org/github.com/NickLarsenNZ/mock-http-response
• https://godoc.org/github.com/NickLarsenNZ/mock-http-response/responders

[NickLarsenNZ]

Here's the current strategy:

https://github.com/NickLarsenNZ/mock-http-response/blob/9f3269dd9d0f5ab83434566b52c8e7ffeed07d7c/mockhttp.go#L41-L81