Update docs of set_if_neq and replace_if_neq

[RobWalt]

Objective

• This PR adds more flexible versions of set_if_neq and replace_if_neq to only compare and update certain fields of a components which is not just a newtype
• Update docs of set_if_neq and replace_if_neq #12919 (comment) gave a good solution to the original problem, so let's update the docs so that this is easier to find

Solution

• Add set_if_neq_with and replace_if_neq_with which take an accessor closure to access the relevant field

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In a recent project, a scenario emerged that required careful consideration regarding change detection without compromising performance. The context involves a component that maintains a collection of Vec<Vec2> representing a horizontal surface, alongside a height field. When the height is updated, there are a few approaches to consider:

1. Clone the collection of points to utilize the existing set_if_neq method.
2. Inline and adjust the set_if_neq code specifically for this scenario.
3. (Consider splitting the component into more granular components.)

It's worth noting that the third option might be the most suitable in most cases.

A similar situation arises with the Bevy internal Transform component, which includes fields for translation, rotation, and scale. These fields are relatively small (Vec3 or Quat with 3 or 4 f32 values), but the creation of a single pointer (usize) might be more efficient than copying the data of the other fields. This is speculative, and insights from others could be valuable.

Questions remain:

• Is it feasible to develop a more flexible API, and what might that entail?
• Is there general interest in this change?

There's no hard feelings if this idea or the PR is ultimately rejected. I just wanted to put this idea out there and hope that this might be beneficial to others and that feedback could be valuable before abandoning the idea.

[SkiFire13]

When the height is updated, there are a few approaches to consider:

Note that there's another alternative which is also very similar to the approach proposed in the PR. You can use map_unchanged to perform the mapping step and then call set_if_neq on the mapped Mut.

In fact I'm pretty sure (haven't actually tried though) that you can implement set_if_neq_with as self.map_unchanged(accessor_f).set_if_neq(value)

[RobWalt]

Thanks for the hint, I didn't know that! Then maybe that should be the way to go and there's no real need for set_if_neq_with. It might make sense to update the docs to point other people to this solution here, though. I'll do that once I find the time

[alice-i-cecile]

@SkiFire13 can I get your review here?

[alice-i-cecile]

Thank you both: this is a helpful note.