Document support for Unsized types

[jswrenn]

With 0.8, zerocopy will have some support for unsized types. However, unsized types are awkward to use in Rust, and we should be extra mindful about documenting how zerocopy can be used with them. This will probably require both specialized documentation on individual methods, and overview documentation on a crate-level.

[joshlf]

Closed in #1273