Add guidance on #[must_use] to engine style guide

> > possibly must_use
>
> I personally think this is a tool to be used reactively to solve specific problems, rather than something to add reflexively to every constructor. It adds restrictions to actions that are generally harmless at the cost of code volume / legibility (constructors like this are very common) and the time cost of suggesting / discussing / changing this every time it comes up in review ... which will happen a lot if this pr meets our criteria for `must_use`.

_Originally posted by @cart in https://github.com/bevyengine/bevy/issues/5466#issuecomment-1196305221_

This is a sensible take, but should be noted in the [Engine Style Guide](https://github.com/bevyengine/bevy/blob/main/.github/contributing/engine_style_guide.md).