Document DriverMode and it's impact on drivers more thoroughly

[MabezDev]

Some of this content is likely to end up in the book, probably in esp-rs/book#179 (whatever we decide to call it eventually). So be sure to sync there before tackling this.

• Document the trait itself, with code examples
• Document the into_{async|blocking} functions on drivers
  • Explain what it does. I.e installs/uninstalls an interrupt handler etc.