[w3c/screen-orientation] Definition of default orientation (#150)

Currently default orientation is defined with the following:  

```
        A <a>document</a>'s <dfn>default orientation</dfn> is the set of
          orientations to which the screen orientation is locked when it is not
          explicitly locked by this API or any other means.
```
It would be great to get some clarification on why default orientation is a set?  From looking at the android docs it looks like the system decides on one orientation if none is specified.

@mounirlamouri Could you please explain this further?

Relates to https://github.com/w3c/screen-orientation/issues/104

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