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. 
This should only add parameter 1. There is no parameter 2 (remember, zero-based) for `gettransaction`.