docs(ngClass): Explain all 3 ways how to use ngClass

Closes angular#6324.

Author: bradw2k

src/ng/directive/ngClass.js

@@ -71,6 +71,18 @@ function classDirective(name, selector) {
  * The `ngClass` directive allows you to dynamically set CSS classes on an HTML element by databinding
  * an expression that represents all classes to be added.
  *
+ * The directive operates in three different ways, depending on which of three types the expression
+ * evaluates to:
+ *
+ * 1. If the expression evaluates to a string, the string should be one or more space-delimited class
+ * names.
+ *
+ * 2. If the expression evaluates to an array, each element of the array should be a string that is
+ * one or more space-delimited class names.
+ *
+ * 3. If the expression evaluates to an object, then for each key-value pair of the
+ * object with a truthy value the corresponding key is used as a class name.
+ *
  * The directive won't add duplicate classes if a particular class was already set.
  *
  * When the expression changes, the previously added classes are removed and only then the