document grid wrapping behavior more explicitly

Created by: cvrebert

For example, that the third column here will wrap onto a new line:

<div class="row">
  <div class="col-xs-6">col-xs-6</div>
  <div class="col-xs-6">col-xs-6</div>
  <div class="col-xs-6">col-xs-6</div>
</div>