Clarify Documentation

[dbolesta]

Cleans up a bit of the img_width_and_height doc to make things clearer.

• Removes a reference definition that was not being used anywhere in the document
• Adds a reference link to clarify the "padding-top hack", which is not common knowledge, and provides further context to what a "padding-top hack" is and how it works.
• Fixes an error where the word "container" was places inside the small css snippet in error.overflow: hidden container makes it seems like container is a CSS value, when really what is means is "a container with overflow: hidden CSS rule applied to it".