v4 Docs: Update outdated ARIA/PF link, expand color contrast explanat…

…ion in `accessibility.md` (#36492)

PF link is backport of #36489
change in `accessibility.md` is to bring this in line with existing v5 prose

site/content/docs/4.6/components/dropdowns.md

@@ -16,7 +16,7 @@ If you're building our JavaScript from source, it [requires `util.js`]({{< docsr
 
 ## Accessibility
 
-The [<abbr title="Web Accessibility Initiative">WAI</abbr> <abbr title="Accessible Rich Internet Applications">ARIA</abbr>](https://www.w3.org/TR/wai-aria/) standard defines an actual [`role="menu"` widget](https://www.w3.org/WAI/PF/aria/roles#menu), but this is specific to application-like menus which trigger actions or functions. <abbr title="Accessible Rich Internet Applications">ARIA</abbr> menus can only contain menu items, checkbox menu items, radio button menu items, radio button groups, and sub-menus.
+The [<abbr title="Web Accessibility Initiative">WAI</abbr> <abbr title="Accessible Rich Internet Applications">ARIA</abbr>](https://www.w3.org/TR/wai-aria/) standard defines an actual [`role="menu"` widget](https://www.w3.org/TR/wai-aria/#menu), but this is specific to application-like menus which trigger actions or functions. <abbr title="Accessible Rich Internet Applications">ARIA</abbr> menus can only contain menu items, checkbox menu items, radio button menu items, radio button groups, and sub-menus.
 
 Bootstrap's dropdowns, on the other hand, are designed to be generic and applicable to a variety of situations and markup structures. For instance, it is possible to create dropdowns that contain additional inputs and form controls, such as search fields or login forms. For this reason, Bootstrap does not expect (nor automatically add) any of the `role` and `aria-` attributes required for true <abbr title="Accessible Rich Internet Applications">ARIA</abbr> menus. Authors will have to include these more specific attributes themselves.
 

site/content/docs/4.6/getting-started/accessibility.md

@@ -24,7 +24,7 @@ Because Bootstrap's components are purposely designed to be fairly generic, auth
 
 ### Color contrast
 
-Most colors that currently make up Bootstrap's default palette—used throughout the framework for things such as button variations, alert variations, form validation indicators—lead to *insufficient* color contrast (below the recommended [WCAG 2.1 text color contrast ratio of 4.5:1](https://www.w3.org/TR/WCAG/#contrast-minimum)) when used against a light background. Authors will need to manually modify/extend these default colors to ensure adequate color contrast ratios.
+Some combinations of colors that currently make up Bootstrap's default palette—used throughout the framework for things such as button variations, alert variations, form validation indicators—may lead to *insufficient* color contrast (below the recommended [WCAG 2.1 text color contrast ratio of 4.5:1](https://www.w3.org/TR/WCAG/#contrast-minimum) and the [WCAG 2.1 non-text color contrast ratio of 3:1](https://www.w3.org/TR/WCAG/#non-text-contrast)), particularly when used against a light background. Authors are encouraged to test their specific uses of color and, where necessary, manually modify/extend these default colors to ensure adequate color contrast ratios.
 
 ### Visually hidden content