bevy_reflect: Improve `List::push` and `List::pop` docs

[MrGVSV]

How can Bevy's documentation be improved?

The docs on List::push and List::pop should better explain the directionality of those operations, as explained in this comment.

We should direct manual (and internal) implementors to uphold the contract that:

1. "pushing" always appends the element to the end of the list
2. "popping" always removes the element from the end of the list

The docs already do this to an extent, but they could give just a bit more detail and make the guarantee more explicit so implementors don't miss it!

[LinusKall]

I clarified the meaning of first/front element and last/back element with some emphasis. What do you think, should I make a PR? Suggestions?

/// An ordered, mutable list of [Reflect] items. This corresponds to types like [`std::vec::Vec`].
///
/// This is a sub-trait of [`Array`] as it implements a [`push`](List::push) function, allowing
/// it's internal size to grow.
///
/// This trait expects index 0 to contain the _first_ or _front_ element.
/// The _last_ or _back_ element refers to the element with the largest index.
pub trait List: Reflect + Array {
    /// Appends an element to the _back_ of the list.
    fn push(&mut self, value: Box<dyn Reflect>);

    /// Removes the _last_ element from the list and returns it, or [`None`] if it is empty.
    fn pop(&mut self) -> Option<Box<dyn Reflect>>;

    /// Clones the list, producing a [`DynamicList`].
    fn clone_dynamic(&self) -> DynamicList {
        DynamicList {
            name: self.type_name().to_string(),
            values: self.iter().map(|value| value.clone_value()).collect(),
        }
    }
}

[MrGVSV]

Yeah I think that's pretty good. I think I like your choice in extracting that info into the docs for List itself rather than repeating it for each method. I wonder if words like last should backlink to those docs? 🤔

My only other comments are:

• Personally I prefer end over back, but maybe that's just me
• I’d include an additional sentence or two that explicitly call out manual implementors to remind them of these "rules"

If you can, I’d definitely make a PR so we can get more feedback from the community and iterate based off that!

[LinusKall]

I wonder if words like last should backlink to those docs? 🤔

By the way, how do I create backlinks? 🤔

[tim-blackbird]

Personally I prefer end over back, but maybe that's just me

Don't mind personally, but front/back would match the method names and docs for VecDeque so I think we should go with that.