Document var x = await signal returns signal arguments.

[ahmed-tabib]

The await keyword being able to return signal arguments is not documented anywhere. There is also a special behavior, where if the signal returns more than one argument the await keyword returns an Array with the arguments in order.

• Documented the behavior in the "GDScript Reference" page, under pre-existing section "Awaiting signals or coroutines", and added some tested usage examples.
• Mentioned the behavior and added a reference in the "Using Signals" page, under a new section "Awaiting signals", near the end.

This PR addresses the following issue: #8393