Document how proposals work

Specifically,
- [ ] what is a proposal,
- [ ] who can propose a proposal,
- [ ] what's a review period,
- [ ] who can approve a proposal,
- [ ] when will a proposal execute,
- [ ] what if it fails to execute,
- [ ] how to trigger a retry,
- [ ] how to decline and
- [ ] etc.

Perhaps add to http://docs.bitshares.org/bitshares/user/transactions-proposed.html or http://docs.bitshares.org/bitshares/tutorials/propose-transaction.html.

There are some bugs related though, e.g. https://github.com/bitshares/bitshares-core/issues/146, https://github.com/bitshares/bitshares-core/issues/214, https://github.com/bitshares/bitshares-core/issues/383.

Perhaps this ticket doesn't belong to this repo.

Start with the existing pages at docs.bitshares.org/bitshares/user/transactions-proposed.html and docs.bitshares.org/bitshares/tutorials/propose-transaction.html, then review issues 146, 214, and 383 for proposal behavior and known bugs. Done means documenting the listed proposal questions, including retry and decline behavior, and deciding whether the documentation belongs in this repository.

Written by the indexing model from the issue text.