Fix missing gettransaction entries in rpcclient

When getting a transaction to a watch-only wallet, the gettransaction RPC call should show transaction details (amount, address, received). This won't happen unless the true "includewatchonly" param is true. 

But, when doing 
bitcoin-cli gettransaction "7b38c67e8f32678bd19ad3537b384f7a619ba1e5bdacac983c9a4cc2c16b657e" true

we currently get an error:
error: {"code":-1,"message":"value is type str, expected bool"}

This is the result of the "true" value method not being serialized to json properly in the rpc client due to the gettransaction entry being missing in rpcclient vRPCConvertParams. I believe this was a bug here on at src/rpcclient.cpp:68 https://github.com/bitcoin/bitcoin/commit/0fa2f8899adf8f9f0ead29ba5d708ead6c5d4eaf

I added the entries correctly (for both 1 and 2 arguments) and also updated the rpc help message for gettransaction to let users know about the includeWatchonly param, following the conventions for the other rpc calls.