Buffer usages mismatch check and documentation for mapped_at_creation size requirement.

[Imberflur]

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.

Connections
I based this on the documented validation behavior:

• https://docs.rs/wgpu/0.13.1/wgpu/struct.BufferUsages.html
• https://gpuweb.github.io/gpuweb/#dom-gpudevice-createbuffer

Description
Buffer usage mismatch validation was lost to a refactor. Specifically, this is the check needed when one of the MAP_* usages is enabled. This re-adds the check.

I also added some documentation on the the validation of the buffer size when mapped_at_creation is true.

Testing
A test was added at wgpu/tests/buffer_usage.rs. I confirmed that improper combinations were being allowed via this test and that with the changes they now fail validation.

[cwfitzgerald]

I love the tests!

[Imberflur]

I was looking over this again and realized there is a small issue with the way the tests are set up. Specifically, I tried to reduce the amount of times we need to perform the setup so that it doesn't need to be done before running each case. However, this means with the failing cases we only test that the first item fails! 😭

Maybe I could instead use error scopes to catch the errors so each failure case doesn't need to redo the whole setup?