Better documention for when columns adding up to less than 12

Created by: dmacvicar

We had a commit of code of a form with columns adding up to 9 and we got into a discussion of whether this was ok, or one needed to have an explicit empty last column to add up to 12.

From the docs is hard to infer much:

• The offsetting columns last example adds up to only 9
• Nesting columns section says "Nested rows should include a set of columns that add up to 12."

So, is it OK for the main grid but not for nested grid? Is it OK all the time but not good practice? is it OK and good practice?

Stackoverflow question mentions the 9 column example as evidence: http://stackoverflow.com/questions/19183924/is-it-wrong-for-bootstrap3-columns-to-add-up-to-less-than-12-or-understanding?rq=1