Update docs about set_password and unlock command in cli_wallet

**User Story**
We added "safe password" feature in #1382 for #1171, but some docs haven't been updated, E.G.
- [ ] README of this repository,
- [ ] the CLI cook book, and
- [ ] [Exchange integration guide](https://dev.bitshares.works/en/master/bts_guide/tutorials/exchange_single_node.html) ([source](https://github.com/bitshares/dev.bitshares.works/blob/master/docs/bts_guide/tutorials/exchange_single_node.rst))
- [ ] possibly other docs.

This issue is for working on the docs.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.
- [x] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Review the safe-password changes from #1382, then inspect the repository README, CLI cookbook, and the source for the Exchange integration guide at docs/bts_guide/tutorials/exchange_single_node.rst. Update the documentation for set_password and the unlock command in each relevant location, including any other affected docs; done means the listed guides accurately describe the current commands.

Written by the indexing model from the issue text.