docs: improvement on wasm_bindgen::convert::traits

[Ali Varaste Pour]

Hi,
I was reading the docs for Abi type in FromWasmAbi and IntoWasmAbi traits, and I thought maybe there is room for improvement in their description, e.g., the docs for Abi in FromWasmAbi says:

    /// The wasm ABI type that this converts from when coming back out from the
    /// ABI boundary.

IMO, the above description is a little puzzling. Maybe it could be rewritten to something like this:

    /// The type which wasm ABI provides when comming back from ABI boundary, to be converted to a rust type.

and similarly for IntoWasmAbi,

    /// The type which wasm ABI is provided when crossing the ABI boundary.

This could surely benefit from insights of other users.

[daxpedda]

Improvements to the docs are always welcome.
I'm happy to review a PR.