* Added missing space
* Added missing period in add-borders-around-your-elements.md
* Added missing space in adjust-the-padding-of-an-element.english.md
I was doing this challenge and noticed that there wasn't a space after "transform:scale(2)" in the description which is inconsistent with the other challenges so I changed it to "transform: scale(2)"
* Clarification of the declaration order overriding
Previous text was talking about an override but it may also confuse readers that the override was done in the order of the listing of class names inside the h1.
I also added an extra case to better clarify the situation.
* Small change mentioning the order of declaration
Missed an order of declaration emphasis on my previous commit, adding it on this one.
* User Flow: Information, Articles, Youtube update
Explanation on what is User Flow with youtube videos and articles for reference and/or resource.
* Update of UX Design article
Update the article with more information and more links to articles and books.
* Product Design: Info, Articles, Youtube update
Explanation on what is Product Design with youtube videos and articles for reference and/or resource.
* Update description of Retina Display
As reported in issue #18129, there is a lack of explanation on what a retina display is and some users find it difficult to understand what they are doing exactly, so I added a short explanation on what a High-Resolution Display is and what a Retina Display is.
