[documentation] Clarify getAccounts and new Account

[tcrowe]

What does it do?

#1154 pointed out that it's not clear what web3.eth.getAccounts is doing. This PR tells how web3.eth.personal.newAccount and web3.eth.getAccounts work together and which RPC methods they call. It describes how it's different than web3.eth.accounts.create().

The missing documentation for web3.eth.personal.getAccounts was added.

Related bounty: #2081

Does it work?

This how it's rendered locally. It will look a little different when it gets deployed.

change web3.eth.getAccounts()

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create web3.eth.personal.getAccounts()

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change web3.eth.personal.newAccount()

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change web3.eth.accounts.create()

[coveralls]

Coverage remained the same at 83.071% when pulling 3223b0f on tcrowe:docs-getaccounts-1154 into 1d9f6c0 on ethereum:1.0.

[coveralls]

Coverage remained the same at 83.071% when pulling 3223b0f on tcrowe:docs-getaccounts-1154 into 1d9f6c0 on ethereum:1.0.

[coveralls]

Coverage remained the same at 83.071% when pulling 3223b0f on tcrowe:docs-getaccounts-1154 into 1d9f6c0 on ethereum:1.0.

[coveralls]

Coverage remained the same at 83.071% when pulling 3223b0f on tcrowe:docs-getaccounts-1154 into 1d9f6c0 on ethereum:1.0.

[nivida]

Wow great! Thanks, @tcrowe!