Move v3 .hidden note into Migration docs

docs/components/utilities.md

@@ -234,7 +234,7 @@ Easily clear `float`s by adding `.clearfix` **to the parent element**. Utilizes
 
 ## Hidden content
 
-Hide any HTML element with the `[hidden]` attribute. Previously, v3.x included a `.hidden` class that forced toggled content. However, we removed it due to conflicts with jQuery's `hide()` function. It's taken from [PureCSS](http://purecss.io). While `[hidden]` isn't natively supported by IE9-10, declaring it `display: none` in our CSS gets around that problem.
+Hide any HTML element with the `[hidden]` attribute. It's taken from [PureCSS](http://purecss.io). While `[hidden]` isn't natively supported by IE9-10, declaring it `display: none` in our CSS gets around that problem.
 
 Furthermore, `.invisible` can be used to toggle the visibility of an element, meaning its `display` is not modified and the element can still affect the flow of the document.
 

docs/migration.md

@@ -179,7 +179,7 @@ Note that the changes to the grid breakpoints in v4 means that you'll need to go
 ## Misc notes to prioritize
 
 - Removed the `min--moz-device-pixel-ratio` typo hack for retina media queries
-- Dropped `.hidden` and `.show` because it interferes with jQuery's `$(...).hide()`.
+- Dropped `.hidden` and `.show` because they conflict with jQuery's `$(...).hide()` function.
 - Change buttons' `[disabled]` to `:disabled` as IE9+ supports `:disabled`. However `fieldset[disabled]` is still necessary because [native disabled fieldsets are still buggy in IE11](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/fieldset#Browser_compatibility).
 
 TODO: audit list of stuff in v3 that was marked as deprecated