docs(contrast): improve doc about WCAG contrast algo used in color-yiq()

scss/_variables.scss

@@ -117,7 +117,8 @@ $chevron-icon-active: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000
 // Set a specific jump point for requesting color jumps
 $theme-color-interval:      8% !default;
 
-// The yiq lightness value that determines when the lightness of color changes from "dark" to "light". Acceptable values are between 0 and 255.
+// The contrast ratio to reach against white, to determine if color changes from "light" to "dark". Acceptable values for WCAG 2.0 are 3, 4.5 and 7.
+// See https://www.w3.org/TR/WCAG20/#visual-audio-contrast-contrast
 $yiq-contrasted-threshold:  4.5 !default;
 
 // Customize the light and dark text colors for use in our YIQ color contrast function.

site/docs/4.4/getting-started/theming.md

@@ -195,7 +195,7 @@ Additional functions could be added in the future or your own custom Sass to cre
 
 #### Color contrast
 
-An additional function we include in Boosted is the color contrast function, `color-yiq`. It utilizes the [YIQ color space](https://en.wikipedia.org/wiki/YIQ) to automatically return a light (`#fff`) or dark (`#111`) contrast color based on the specified base color. This function is especially useful for mixins or loops where you're generating multiple classes.
+An additional function we include in Bootstrap is the color contrast function, `color-yiq`. It utilizes the [WCAG 2.0 algorithm](https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-tests) for calculating contrast thresholds based on [relative luminance](https://www.w3.org/WAI/GL/wiki/Relative_luminance) in a `sRGB` colorspace to automatically return a light (`#fff`) or dark (`#111`) contrast color based on the specified base color. This function is especially useful for mixins or loops where you're generating multiple classes.
 
 For example, to generate color swatches from our `$theme-colors` map:
 
@@ -223,6 +223,13 @@ You can also specify a base color with our color map functions:
 }
 {% endhighlight %}
 
+{% capture callout %}
+##### Accessibility
+
+In order to meet [WCAG 2.0 accessibility standards for color contrast](https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html), authors **must** provide [a contrast ratio of at least 4.5:1](https://www.w3.org/WAI/WCAG20/quickref/20160105/Overview.php#visual-audio-contrast-contrast), with very few exceptions.
+{% endcapture %}
+{% include callout.html content=callout type="info" %}
+
 #### Escape SVG
 
 We use the `escape-svg` function to escape the `<`, `>` and `#` characters for SVG background images. These characters need to be escaped to properly render the background images in IE.