Update lockup docs

[jonator]

Summary of Bug

From talking to Dev, I've been made aware that the lockup module docs are out of date. More specifically, it appears there's a contradiction with how tokens are moved from the lockup module back to the user's account.

In the "Locked coins management" section, it says users must withdraw funds from the lockup module once the unbonding period is over. I interpret this to be a user-initiated action.

However, in this "Endblocker" section, it implies that lock statuses are checked at the end of each block and for expired locks, funds are immediately moved back the owning accounts. This scenario seems to be what actually happens based on the frontend behavior and based on what Dev has said.