0.13 migration: Use clippy-approved ReceivedCharacter advice

[rparrett]

And other improvements. I went in there to fix the clippy thing and things sort of snowballed.

• Moved the text to a subsection of the PR that caused the breakage

• Added some context about why it's not a char and what else might be in there.

• Added a sentence about a new alternative to ReceivedCharacter that folks may want to use instead.

• Modified the previous advice so that it is Clippy-approved.

  The previous advice would result in:

  warning: called `.nth(0)` on a `std::iter::Iterator`, when `.next()` is equivalent
     --> src/typing.rs:126:30
      |
  126 |             let Some(char) = event.char.chars().nth(0) else {
      |                              ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try calling `.next()` instead of `.nth(0)`: `event.char.chars().next()`
      |
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_nth_zero
      = note: `#[warn(clippy::iter_nth_zero)]` on by default
  

  Clippy suggests .next(), but based on the winit docs, it seems like .last() might be the way to go to me. In the cases where there's more than one char, folks probably want the last one.

Some of this stuff should probably make its way into Bevy's docs or a bevy issue.

[rparrett]

Opened docs issue here: bevyengine/bevy#12040