Issue #1865 - Clean up RPC help messages

Based on the proposal, update the help message of rpc methods
- strings arguments are in double quotes rather than square brackets
- numeric arguments have no quotes (and no default value)
- optional parameters are surrounded by round brackets
- json arguments are strings but don't use double quotes

Added 3 sections for the details
- Arguments: lists each argument, it's type, required or not, a default, and a description
- Result: The method result, with json format if applicable, type, and a description
- Examples: examples calls using bitcoin-cli and curl for json rpc call

Problems
- maybe this is too verbose
- lines might be too long
- description are not good or complete
- examples may be too much