Documentation/Hint on each function

I think I have suggested renaming `s` and `str` to something more userfriendly but got rejected =/
Can we instead provide documentation with `///` on each of the functions? I think the documentation in the `Elmish.UrlParser` github page is very helpful, so maybe adding it to the function would also provide value...

```
* s combinator for a static string we expect to find in the URL,
* combinator to capture a slash in the URL,
* str combinator to extract a string,
* i32 combinator to attempt to parse an int.
* top combinator that takes nothing.
* remaining combinator to get the remaining parts of the URL
```

I tend to shadow like this to provide clarity:

```F#
// combinator for a static string we expect to find in the URL
let matchString = UrlParser.s
// combinator to extract a string
let parseString = UrlParser.str
// combinator to attempt to parse and Int32
let parseInt32 = UrlParser.i32
```