Fix warnings and errors from beta Rust

[bengsparks]

Motivation

The CI cannot pass due to the marking of two new warnings / errors, namely pattern exhaustiveness (by rustc) and long doc comments (by clippy).

Solution

1. Pattern exhaustiveness

error: unreachable pattern
   --> axum/src/middleware/map_response.rs:281:25
    |
281 |                         Err(err) => match err {}
    |                         ^^^^^^^^ matches no values because `std::convert::Infallible` is uninhabited

Because unreachable pattern is a useful compiler warning, and within all of axum, there are only 4 occurrences, it is simplest to just mark the match arms with #[allow].
Because the examples are separate from the axum lib, it's simplest to just use the same #[allow] in each example's main.rs at a module level.

An alternative would be to add the directive to axum's lib.rs, but that would suppress other instances of this error.
Additionally, there does not seem to be a way to disable more specific instances of unreachable patterns, or to configure this warning's impact from Cargo.toml.

NOTE: It seems that this warning has irked enough people for an exception for empty match arms to be made and nominated for backporting to beta, so hopefully these commits can be reverted soon.

2. clippy::too-long-first-doc-paragraph

error: first doc comment paragraph is too long
   --> axum/src/extract/ws.rs:785:5
    |
785 | /     /// Indicates that an endpoint is terminating the connection because it has received a type of
786 | |     /// data it cannot accept (e.g., an endpoint that understands only text data MAY send this if
787 | |     /// it receives a binary message).
    | |____^

This lint is likely here to stay, so this requires actual changes, which were made in accordance with the fix recommended by the lint's author(s).

[jplatte]

Thanks!