Docs for disabled inline radio/checkboxes

[TheSharpieOne]

Similar to #20608, this ticket aims to address the "inline" radio/checkbox which use form-check-inline. It was not explicitly pointed out in the previous issue, which is my fault. Rather than reopen it, I am making this new ticket.
The disabled styles between the form-check and form-check-inline are inconsistent as can be seen in the image below.

The docs do not clearly indicate how to mark a form-check-inline as disabled. See this pin for the code used in the image: http://codepen.io/TheSharpieOne/pen/LRLErr?editors=1000

Relevant code: https://github.com/twbs/bootstrap/blob/v4.0.0-alpha.4/scss/_forms.scss#L241-L243

[cvrebert]

It would seem that either .form-check-inline is missing the color: $text-muted; of its .disabled .form-check-label counterpart, or that the markup scheme needs tweaking.