Improve documentation of veritcal_rhythm partial.

[JohnAlbin]

The docs for the vertical rhythm partial need some love. Here's the love.

[barraponto]

I remember when I was beginner frontender and @JohnAlbin 's Zen base theme for Drupal and @chriseppstein 's SASS and Compass sped me down the highway of frontend and in-browser design... I just feel happy to see you both working together :)

[JohnAlbin]

I finally had to go through the vertical_rhythm partial line-by-line to try to understand it. This commit is basically my notes to try to make sense of the partial.

The partial is actually pretty good. Not perfect, but like 90% good. :-)

I've got a boat load of changes I want to improve this partial (including decent rem support), but since my doc changes touch almost every line, it would be easier to get this in first and save the merge conflicts.

[chriseppstein]

Excellent. Thank you.