Another grid update

* Update docs to include table of grid levels and behaviors
* Change from .col-md to .col-sm to match existing size variables we use elsewhere
* Keep .col-lg as-is though for simplicity as we won't likely add a fourth level of the grid
* Combine responsive .container breakpoints with grid breakpoints in grid.less
* Other misc docs updates

docs/assets/css/docs.css

@@ -520,6 +520,7 @@ body {
 
 /* Utility classes table
 ------------------------- */
+.bs-table th small,
 .responsive-utilities th small {
   display: block;
   font-weight: normal;