chore(slide-toggle): add initial documentation of properties

[devversion]

• Adds a basic documentation of the public properties like in the checkbox component.

Note: Still documenting tabIndex (regardless of being filtered), because this would allow us in the future to easily remove the filtering and just have it documented. Also just for consistency.

@jelbourn The documentation might be sometimes very bare, but for now it should be good (most of the stuff is similar in the md-checkbox)

[jelbourn]

LGTM, thanks!

[jelbourn]

@devversion can you rebase this?

[devversion]

@jelbourn Done

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