Add documentation for containerStyle prop

[randycoulman]

Support (and documentation) for containerStyle was added to SegmentedControls in 5d15bd0. However, the documentation for it was removed in #54 (probably by accident).

This PR adds back the documentation for containerStyle and adds additional information about what all three style props do.

The documentation for optionStyle and optionContainerStyle was copied from RadioButtons above, so may be redundant, but there’s no other place to more fully document containerStyle, because that style doesn’t apply to RadioButtons; it only applies to SegmentedControls.

[ArnaudRinquin]

Thanks!