First-character argument flag is undocumented and behaves strangely when colliding with default options

Consider the function signature:

```
def main(headless=False):
# Construct GUI
if not headless:
widget.show()
app.exec()

if __name__ == '__main__': fire.Fire(main)
```

After reading through the user guide for fire, I would expect `python myFile -h` to produce the help signature for this file. However, it runs the script with `headless = True`.

While in retrospect I suppose this makes sense, I would like some way of telling fire to disallow shortcut flags if they override user-expected options like --[i]nteractive, --[h]elp, etc. In those cases, the user can be forced to say `python myFile --headless` for the desired behavior.

Moreover, if my script looked like this:
```
def main(headless=False, hArg=True):
# Construct GUI
if not headless:
widget.show()
app.exec()

if __name__ == '__main__': fire.Fire(main)
```
and I ran the same line of code, fire would appropriately err out. This is also acceptable in my opinion.

More importantly, the first-character flag behavior should at the very least be documented. I read through https://github.com/google/python-fire/blob/master/docs/guide.md and couldn't find it there.