Fix and improve doc on Gui.__init__

[FlorianJacta]

From a user:

The documentation of Gui.init(), specifically concerning parameter env_filename, could use a couple of fixes:

1. The table's default column has None listed, but in the description it notes that the default = taipy.gui.env.
2. Additional explanation that the env_filename will ALWAYS override over any passed kwargs in run(). To the end user this is very unclear. Without an explanation, one would logically expect that the following code would use the kwargs passed to run():

app = Gui(pages=pages, env_filename='taipy.gui.env') # containing run_browser=True
if __name__ == '__main__':
    app.run(run_browser=False)

In reality, the browser settings are overridden by the env-file.

Debugging this was especially difficult without knowing that the default of env_filename is taipy.gui.env, and that app = Gui() was in fact still loading the env-file