docs: Add more overview documentation to Buffer and usages.

[kpreid]

Description
I thought that “how do I actually use a Buffer” could use some more explanation. Added:

• Mention that buffers are untyped allocations of bytes.
• Mention that multiple things can be stored in one buffer.
• Summarize all the ways provided by wgpu to write to a buffer.
• Mention 2 reasons why you should keep your BufferUsages small.

Testing
cargo doc --open && 👀

Squash or Rebase?
One commit!

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[cwfitzgerald]

Nits but nice improvements!

[cwfitzgerald]

Suggested change

	/// Specifying only usage the application will actually perform may increase performance.
	/// Additionally, on the WebGL backend, there are restrictions on [`BufferUsages::INDEX`];
	/// see [`DownlevelFlags::UNRESTRICTED_INDEX_BUFFER`] for more information.
	/// Specifying only usages that the application will actually use may theoretically increase performance.
	/// Additionally, on the WebGL backend, there are restrictions on [`BufferUsages::INDEX`];
	/// see [`DownlevelFlags::UNRESTRICTED_INDEX_BUFFER`] for more information.

[kpreid]

"Usages that the application will [] use" is poor English to my ear; it’s wrong like like saying "consumptions that you will eat” would be wrong. I’ll add the plural, but “perform” is appropriate here either way. If you still disagree, please let me know what distinction you are aiming at here and I’ll find another phrasing.

[cwfitzgerald]

Suggested change

	/// Specifying only usage the application will actually perform may increase performance.
	/// Additionally, on the WebGL backend, there are restrictions on [`BufferUsages::INDEX`];
	/// see [`DownlevelFlags::UNRESTRICTED_INDEX_BUFFER`] for more information.
	/// Specifying only usages that the application will actually use may theoretically increase performance.
	/// Additionally, on the WebGL backend, there are restrictions on [`BufferUsages::INDEX`];
	/// see [`DownlevelFlags::UNRESTRICTED_INDEX_BUFFER`] for more information.

[kpreid]

Changed the plural, but I don’t see how adding “theoretically” does any work that “may” does not. Do you see this statement as too strong? If so, can you describe what the distinction is?

[ErichDonGubler]

Highly nitpicky stuff.

[ErichDonGubler]

style: You refer to every other method/function using parens. Let's do the same here.

[kpreid]

Done.

However! The entire rest of the project generally doesn’t use parens, so arguably this is making it more inconsistent. I'd be up for adding them across the whole project. What do you think?