Fixed critical inaccuracies about modifiable code

[cmaggiulli]

I made similar changes in the 6.2.x branch. Basically I made the following three changes in the first commit.

1. Most importantly, I corrected an inaccuracy regarding which three classes are modifiable. ENTITY_NAMEModelImpl is not modifiable, ENTITY_NAMEimpl is. Also while correcting that I removed the * notation and added the _ notation that is used in the rest of the documentation.
2. When applicatable I changed "customizable" to "modifiable" because that is how it is referred to in the JavaDocs and block comments
3. Figure 4 says that the EntityImpl class is the extension point for the service layer. Its not, its the extension point for the model layer

In support of my pull request I present the following evidence. Below are block comments from the master branch source code.

FeedModelImpl.java

	/*
	 * NOTE FOR DEVELOPERS:
	 *
	 * Never modify or reference this class directly. All methods that expect a feed model instance should use the {@link Feed} interface instead.
	 */

Not the "Never Modify" in the comments. In the master branch source code I cannot find any _Entity_Impl block comments presumably because those files werent generated by Service Builder in the core portal code. However here is an example from a generated _Entity_Impl class from a Liferay MVC Portlet with Service Builder.

	/*
	 * NOTE FOR DEVELOPERS:
	 *
	 * Never reference this class directly. All methods that expect a order model instance should use the {@link com.latham.inquiry.model.Order} interface instead.
	 */

You still are instructed not to reference this class (instead referencing its corresponding interface) but you are not instructed to not modify it... because its modifiable.

This also serves as justification for the verbiage change from custom to modify, and as evidence that the _Entity_Impl class is a part of the model layer, not service layer

In the second commit I reorganized the page a bit to put like things closer together. I also added a "Resources" list element in the generated code list because.... well.... it is xml/sql code that is generated as a result of running Service Builder. Modifications to those files can and will effect the code that is generated in many instances. Also I added the appropriate related topi links at the bottom