Proofread and update `membership` module page

[xuniq]

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_rock/membership/
SME: @ yngvar-antonsson

Details

Module README: https://github.com/tarantool/membership#readme
API: https://github.com/tarantool/membership/blob/7a8781640e54b75d849753a7ec23806575a7ebdc/membership.lua

Definition of done

• Update overview sections according to README file
• Check and update API description