Review and improve form field labels and description

Problem/Motivation

The form fields are currently a bit confusing to me, and I've used the module a bit and even maintained it.

Steps to reproduce

1. Visit /admin/config/people/user-expire
2. Be confused

Proposed resolution

Remaining tasks

review the purpose of each field
propose alternate labels and descriptions as needed
make changes in an MR
release

User interface changes

New labels on many admin fields and some help texts as well as necessary.

API changes

n/a

Data model changes

n/a