Context: improve javadoc

1) Mention that it is OK for the passed pluginIndex to be null.

2) Remove the "NB" block about how PluginIndex#discover() "may" still be
   called. Actually, it is definitely always called, unless a subclass
   of PluginIndex is passed. And the attached PluginFinder will be used.
   We may still want to mention this, maybe, but if so it should be done
   in the main Context ctor rather than one of the convenience ones.

Author: ctrueden

src/main/java/org/scijava/Context.java

@@ -125,20 +125,15 @@ public Context(final Collection<Class<? extends Service>> serviceClasses) {
 	/**
 	 * Creates a new SciJava application with the specified PluginIndex. This
 	 * allows a base set of available plugins to be defined, and is useful when
-	 * plugins that would not be returned by the PluginIndex's PluginFinder are
-	 * desired.
-	 * <p>
-	 * NB: the {@link PluginIndex#discover()} method may still be called, adding
-	 * additional plugins to this index. The mechanism of discovery should be
-	 * configured exclusively through the attached
-	 * {@link org.scijava.plugin.PluginFinder}.
-	 * </p>
+	 * plugins that would not be returned by the {@link PluginIndex}'s
+	 * {@link org.scijava.plugin.PluginFinder} are desired.
 	 * 
 	 * @param pluginIndex The plugin index to use when discovering and indexing
 	 *          plugins. If you wish to completely control how services are
 	 *          discovered (i.e., use your own
 	 *          {@link org.scijava.plugin.PluginFinder} implementation), then you
-	 *          can pass a custom {@link PluginIndex} here.
+	 *          can pass a custom {@link PluginIndex} here. Passing null will
+	 *          result in a default plugin index being constructed and used.
 	 */
 	@SuppressWarnings("unchecked")
 	public Context(final PluginIndex pluginIndex) {
@@ -166,7 +161,8 @@ public Context(final PluginIndex pluginIndex) {
 	 *          plugins. If you wish to completely control how services are
 	 *          discovered (i.e., use your own
 	 *          {@link org.scijava.plugin.PluginFinder} implementation), then you
-	 *          can pass a custom {@link PluginIndex} here.
+	 *          can pass a custom {@link PluginIndex} here. Passing null will
+	 *          result in a default plugin index being constructed and used.
 	 */
 	public Context(final Collection<Class<? extends Service>> serviceClasses,
 		final PluginIndex pluginIndex)