GuiNewWindow Improve Documentation

[jgehrig]

Usage of the command is unclear (at least to me):

							*GuiNewWindow(opts)*
|GuiNewWindow()| creates a new window with the given map of arguments.
Supported arguments are:

The following parameters describe the fields in {opts}.

Parameters: ~
	{server} (optional, string): Server to connect to for the new window.
	{nvim} (optional, string): Path to nvim instance.

I think the command is used like this? :GuiNewWindow({"server": "arg", "nvim": "arg"}).

We should be explicit about the usage. If it is not dead-simple/obvious, we will get bug reports.

We also need to allow :GuiNewWindow() without any arguments. I suspect this is how most users will invoke the command?

[jgehrig]

We should also add something to the Wiki: https://github.com/equalsraf/neovim-qt/wiki/Configuration-Options