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