Bug 19886 setProjectJavadocLocation() only in internal package [api] [javadoc]
If You want to set the project javadoc location by API so You have to use an internal class, there is no public interface. see class org.eclipse.jdt.internal.corext.javadoc.JavaDocLocations (Otherwise the method setLibraryJavadocLocation() is public at org.eclipse.jdt.ui.JavaUI - thats ok.)
org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/JavaUI.java
*
	 * @see ITypeHierarchyViewPart
	 * @see org.eclipse.ui.IWorkbenchPage#findView(java.lang.String)
	 * @see org.eclipse.ui.IWorkbenchPage#showView(java.lang.String)
	 */ 
	public static final String ID_TYPE_HIERARCHY= 		"org.eclipse.jdt.ui.TypeHierarchy"; 
	/**
	 * The id of the Java Browsing Perspective
	 * (value <code>"org.eclipse.jdt.ui.JavaBrowsingPerspective"</code>).
	 * 
	 * @since 2.0
	 */
	public static String ID_BROWSING_PERSPECTIVE= "org.eclipse.jdt.ui.JavaBrowsingPerspective"; 
	/**
	 * The view part id of the Java Browsing Projects view
	 * (value <code>"org.eclipse.jdt.ui.ProjectsView"</code>).
	 * 
	 * @since 2.0
	 */
	public static String ID_PROJECTS_VIEW= "org.eclipse.jdt.ui.ProjectsView"; 
	/**
	 * The view part id of the Java Browsing Packages view
	 * (value <code>"org.eclipse.jdt.ui.PackagesView"</code>).
	 * 
	 * @since 2.0
	 */
	public static String ID_PACKAGES_VIEW= "org.eclipse.jdt.ui.PackagesView"; 
	/**
	 * The view part id of the Java Browsing Types view
	 * (value <code>"org.eclipse.jdt.ui.TypesView"</code>).