Fix/merge advice on low percentage progress bars

- actually add the ``min-width`` extra style (and use ``em`` rather than
``px``) to the 0% and 2% example
- remove the unnecessarily redundant sentence in the "With labels"
section
- remove the unnecessary heading/section specifically for low percentage
case, make it flow naturally as part of the "With labels" section
Filed http://openradar.appspot.com/19208483 for #12536
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Merge pull request #15334 from patrickhlauke/progress-low-percentages

Fix/merge advice on low percentage progress bars
Merge pull request #15337 from twbs/patrickhlauke

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Specify `img` dimensions in the HTML.
Tweak colors to improve accessibility.
Run `grunt`.