[CSS21] Block-level element vs. block-like container

Two comments:

   1. It should probably be block-level box, not block-level element

   2. We have a lot of places where we list out specific display types,
      and mostly what we mean is a container box that behaves like a
      block. These lists get out-of-sync as we add features or correct
      errors. It would help if we defined a new term for them. I think
      it's important to do this in 2.1 so that this vocabulary is
      available for all CSS3 modules.

~fantasai