samd21g18a-0.2.1 doesn't have any documentation.
!! !DEPRECATED: use https://crates.io/crates/atsamd21-hal instead (or one of the specific device crates implemented with it)
Peripheral access API for ATSAMD21G18A microcontrollers (generated using svd2rust v0.11.1). Modified by hand by David McGillicuddy (contact@djmcgill.co.uk)
You can find an overview of the API here.
You can find the specific rustdocs of this crate here.