Emphasize that Request extractor goes last

[bittrance]

Motivation

The error message you get when passing an invalid fn or closure to from_fn is rather far removed from the actual error. The itemized list in https://docs.rs/axum/latest/axum/middleware/fn.from_fn.html makes no mention of the limitation that Request must be the last extractor. This is only mentioned in passing in the auth middleware example a way down the page.

Solution

Given that putting extractors in the wrong order risks causing a lot of confusion, this PR promotes the ordering requirement to the top of from_fn docs so that it is easier to note.

[mladedav]

Thanks for improving the docs!

[mladedav]

Thank you