Missing documentation on features

[VorpalBlade]

The Cargo.toml features of this crate appear to be undocumented. For example:

• What is the difference between alloc in this crate, in esp-idf-svc and alloc_handler in esp-idf-sys?
• There appear to be embassy related features, is that a work in progress or ready?
• The critical-section feature, when do I need it?

I'm new to embedded Rust, and I'm finding the three idf crates (hal, sys and svc) quite confusing. Some brief overview documentation would go a long way.

[Vollbrecht]

1. even though most people use esp-idf-svc/hal with the standard library we implemented it in a way so it can be used in a no_std environement. So when used there, it gates the parts that still need a allocator. The user than can provide its own allocator or uses it from std.
2. We are using some embassy stuff, but also implement some other async related stuff esp specific - and its all working. But this is all not recommended to use if you are just beginning working with embedded rust.
3. https://en.wikipedia.org/wiki/Critical_section

[ivmarkov]

Closing in favor of #258