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
return fgSharedImages;
	}
	 
	/**
	 * Creates a selection dialog that lists all packages of the given Java project.
	 * The caller is responsible for opening the dialog with <code>Window.open</code>,
	 * and subsequently extracting the selected package (of type
	 * <code>IPackageFragment</code>) via <code>SelectionDialog.getResult</code>.
	 * 
	 * @param parent the parent shell of the dialog to be created
	 * @param project the Java project
	 * @param style flags defining the style of the dialog; the valid flags are:
	 *   <code>IJavaElementSearchConstants.CONSIDER_BINARIES</code>, indicating that 
	 *   packages from binary package fragment roots should be included in addition
	 *   to those from source package fragment roots;
	 *   <code>IJavaElementSearchConstants.CONSIDER_REQUIRED_PROJECTS</code>, indicating that
	 *   packages from required projects should be included as well.
	 * @param filter the initial pattern to filter the set of packages. For example "com" shows 
	 * all packages starting with "com". The meta character '?' representing any character and 
	 * '*' representing any string are supported. Clients can pass an empty string if no filtering 
	 * is required.
	 * @return a new selection dialog
	 * @exception JavaModelException if the selection dialog could not be opened
	 * 
	 * @since 2.0
	 */
	public static SelectionDialog createPackageDialog(Shell parent, IJavaProject project, int style, String filter) throws JavaModelException {
		Assert.isTrue((style | IJavaElementSearchConstants.CONSIDER_BINARIES | IJavaElementSearchConstants.CONSIDER_REQUIRED_PROJECTS) ==
			(IJavaElementSearchConstants.CONSIDER_BINARIES | IJavaElementSearchConstants.CONSIDER_REQUIRED_PROJECTS));
		IPackageFragmentRoot[] roots= null;