Update to the `Image.SMILE` description

[microbit-carlos]

Reported by @microbit-lucy from a support ticket.

Image.SMILE only includes the mounth:

So we might update the description to something like "Smiling mouth image" (originally suggested by @jaustin):

@microbit-grace is this something you could look into?

[microbit-robert]

This is now on beta and will be included in the next release. Thanks @microbit-grace.