Document part of bevy_ecs::Commands

[tangmi]

Here is a first pass at adding some clarification to Commands, which is a type that most folks need to use and can sometimes be confusing. This is not meant to be a "final draft" but are intended just to help answer some common questions for new users before bevy has more thorough docs.

I've specifically tried to address which methods should be called with bundles vs single components. This changeset renames some usages of components to bundle.

I'd also like to document the following, but need some further guidance for:

• insert_local_resource: I'm unsure what SystemId is in relation to Resources (is it just like a namespace for archetypes?)
• despawn: I'd like to clarify what "other considerations" the existing comment is referring to (currently documented as: Despawns only the specified entity, ignoring any other consideration.)

[mockersf]

Since latest rust version you can use easy intra doc links: https://doc.rust-lang.org/stable/rustdoc/linking-to-items-by-name.html

Could you link all items that make sense? (like the "See something")

[mockersf]

for despawn, I think the "considerations" are about hierarchy. despawn will just despawn the entity, not its children, unlike despawn_recursive

[tangmi]

Is this correct? I'm basing this off of the method signature, but World::spawn_batch's docs seem to imply that I should be an iterator of Component rather than Bundle

[tangmi]

@cart I wonder if you can help clarify this for me?

[cart]

insert_local_resource: I'm unsure what SystemId is in relation to Resources (is it just like a namespace for archetypes?)

"local resources" are resources created for a specific system. SystemId is a unique identifier for a system.

despawn: I'd like to clarify what "other considerations" the existing comment is referring to (currently documented as: Despawns only the specified entity, ignoring any other consideration.)

I'm guessing the author intended "other considerations" to be "children" (to contrast with the despawn_recursive command). I personally think we should just cut out that phrasing. We should just describe what despawning a specified entity does. We don't need to care about describing what it doesn't do.

[tangmi]

Since latest rust version you can use easy intra doc links: https://doc.rust-lang.org/stable/rustdoc/linking-to-items-by-name.html

Could you link all items that make sense? (like the "See something")

It may be worth adding #![deny(broken_intra_doc_links)] whenever #![warn(missing_docs)] is considered to be added.