Document when types are not WebGPU types.

[kpreid]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file. Skipping this

Connections
This is a probably-less-interesting side branch from #2653 where I added documentation linking to the WebGPU specification to many types.

Description
This change documents types that don't correspond to the WebGPU specification, so readers can know they are unique to wgpu.

I also changed the wording in BufferSlice to be more consistent with other documentation.

Testing
Only documentation was changed. I manually examined the rustdoc output.

[codecov-commenter]

Codecov Report

Merging #3235 (9b1c41a) into master (d4b1d57) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3235      +/-   ##
==========================================
- Coverage   64.64%   64.63%   -0.01%     
==========================================
  Files          82       82              
  Lines       39501    39501              
==========================================
- Hits        25534    25532       -2     
- Misses      13967    13969       +2     

Impacted Files	Coverage Δ
wgpu/src/lib.rs	50.33% <ø> (ø)
wgpu/src/util/mod.rs	12.50% <ø> (ø)
wgpu-hal/src/dx12/descriptor.rs	87.62% <0.00%> (-0.50%)	⬇️
wgpu-core/src/hub.rs	60.67% <0.00%> (-0.16%)	⬇️

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[cwfitzgerald]

The only thing I worry about is that people might interpret this as them being native features, not just that they're unique to wgpu.

[kpreid]

Hmm. For that concern, what do you think of something like

This type is unique to the Rust API of wgpu. In the WebGPU specification, {explain the corresponding thing} instead.

The idea being to emphasize that this is a “Rust bindings” thing.

[cwfitzgerald]

That looks great

[cwfitzgerald]

For above change.

[kpreid]

Done. For example, Operations' documentation now contains

This type is unique to the Rust API of wgpu. In the WebGPU specification, separate loadOp and storeOp fields are used instead.