Merge "Update unclear Fragment Javadoc." into lmp-dev

 * activity: if the activity is stopped, no fragments inside of it can be

 * started; when the activity is destroyed, all fragments will be destroyed.

 *

 * <p>All subclasses of Fragment must include a public empty constructor.

 * <p>All subclasses of Fragment must include a public no-argument constructor.

 * The framework will often re-instantiate a fragment class when needed,

 * in particular during state restore, and needs to be able to find this

 * constructor to instantiate it.  If the empty constructor is not available,

 * a runtime exception will occur in some cases during state restore.

 * constructor to instantiate it.  If the no-argument constructor is not

 * available, a runtime exception will occur in some cases during state

 * restore.

 *

 * <p>Topics covered here:

 * <ol>