Move help block docs up page, add `.help-block` to validation example

Created by: zacechola

Closes #16805 (closed).

• Moves "Help text" section above validation section to improve the flow, so that validation isn't the first reference to a .help-block class.
• Added an example of .help-block in the .has-success section.

I think one example should suffice, but if not, we can add it to each example.