Bootstrap: Add uncolored .btn example to docs.

[Igcorreia]

Just notice that we have a new or another color for buttons:

<button class="btn">link</button>

This button is like midtone grey. This one should be shown in the examples:

https://v4-alpha.getbootstrap.com/components/buttons/

[Studio384]

That's not supposed to be used if I remember correctly. That is just the basic .btn-style that is used as a "reset style" (that's not the right word) for the other .btn-*-classes.

[Igcorreia]

@Studio384 it is a style do. :)

[Studio384]

It's not, it's like saying that .btn-primary on itself is a correct style as well.

[mdo]

.btn by itself is incomplete—it's what we call the base class. Base classes require modifier classes to apply additional styles on top of the shared ones in the base class. As such, there's nothing else to document here at this time.