Update API JavaDocs for clarification (#4180)

* Fix javadoc for IGridBlock.getMachineRepresentation()

Javadoc should state proper semantics for that method, and telling people to return null there is a sure crash. #4178

* Update IPart interface javadoc.

Update the javadoc to state that functional parts should not be allowed to be placed on dense cables.

src/api/java/appeng/api/networking/IGridBlock.java

@@ -123,7 +123,7 @@ public interface IGridBlock
 	/**
 	 * Determines what item stack is used to render this node in the GUI.
 	 *
-	 * @return the render item stack to use to render this node, null is valid, and will not show this node.
+	 * @return the render item stack to use to render this node. Returning an empty ItemStack will not show this node.
 	 */
 	@Nonnull
 	ItemStack getMachineRepresentation();

src/api/java/appeng/api/parts/IPart.java

@@ -291,6 +291,9 @@ public interface IPart extends IBoxProvider, ICustomCableConnection
 
 	/**
 	 * Used to determine which parts can be placed on what cables.
+	 * 
+	 * Dense cables are not allowed for functional (getGridNode returns a node) parts.
+	 * Doing so will result in crashes.
 	 *
 	 * @param what placed part
 	 *
@@ -365,4 +368,4 @@ public interface IPart extends IBoxProvider, ICustomCableConnection
 		return null;
 	}
 
-}
+}