Update docs about set_password and unlock command in cli_wallet

[abitmore]

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 (source)
• 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.

• 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