Update docs to clarify "python" vs "pythonArgs" arguments in launch.json

[luabud]

This is a follow up item for #4161.

In this section of our docs we say that if users wish to pass arguments to the Python interpreter they can use an array of values for the "python" property. We changed that so that python only accepts a string, and we added a new setting called "pythonArgs" to pass arguments to the interpreter.

More details here:
microsoft/vscode-python#12462
microsoft/vscode-python#13218

[gregvanl]

@luabud Documentation PRs welcome.