Describe the pkgver comparison algorithm in prose

Currently the alpm-pkgver specification (added in !3 (merged)) is missing a description of the comparison behavior.

A description should be added based on the behavior implemented in https://gitlab.archlinux.org/archlinux/alpm/alpm/-/blob/0a535a07b3a884d34e4e45dba26a202a0138b2f2/alpm-types/src/version.rs#L175-412.