Add guidance on #[must_use] to engine style guide

[alice-i-cecile]

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 #5466 (comment)

This is a sensible take, but should be noted in the Engine Style Guide.