Javadoc and Formatting fixes. (#619)

Javadoc and Formatting fixes.

Also, extremely minor code changes which have been tested.
This commit is only part one of two commits that aim to fix problems with formatting in our project. In part two I will modify the Google Java Style Guide (since it closely matches our code style) for our project so there is guidance on how to format and document. 

* Updated PlotSquared URL
* Removed plugin acronyms
* Fixed a typo
* Fixed grammar
* Use modern block id's
* Update YouTube video URL

worldedit-core/src/main/java/com/sk89q/worldedit/session/ClipboardHolder.java

@@ -49,9 +49,11 @@ public class ClipboardHolder {
 
     /**
      * Get the clipboard.
+     *
      * <p>
      * If there is a transformation applied, the returned clipboard will
      * not contain its effect.
+     * </p>
      *
      * @deprecated FAWE supports multiple loaded schematics {@link #getClipboards()}
      * @return the clipboard

worldedit-core/src/main/java/com/sk89q/worldedit/session/request/Request.java

@@ -36,11 +36,16 @@ public final class Request {
 
     private static final CleanableThreadLocal<Request> threadLocal = new CleanableThreadLocal<>(Request::new);
 
-    private @Nullable World world;
-    private @Nullable Actor actor;
-    private @Nullable LocalSession session;
-    private @Nullable EditSession editSession;
-    private @Nullable Extent extent;
+    @Nullable
+    private World world;
+    @Nullable
+    private Actor actor;
+    @Nullable
+    private LocalSession session;
+    @Nullable
+    private EditSession editSession;
+    @Nullable
+    private Extent extent;
     private boolean valid;
 
     private Request() {
@@ -55,7 +60,8 @@ public final class Request {
      *
      * @return the world, which may be null
      */
-    public @Nullable World getWorld() {
+    @Nullable
+    public World getWorld() {
         return world;
     }
 
@@ -72,10 +78,17 @@ public final class Request {
         this.extent = extent;
     }
 
-    public @Nullable Extent getExtent() {
-        if (extent != null) return extent;
-        if (editSession != null) return editSession;
-        if (world != null) return world;
+    @Nullable
+    public Extent getExtent() {
+        if (extent != null) {
+            return extent;
+        }
+        if (editSession != null) {
+            return editSession;
+        }
+        if (world != null) {
+            return world;
+        }
         return null;
     }
 
@@ -93,7 +106,8 @@ public final class Request {
      *
      * @return the session, which may be null
      */
-    public @Nullable LocalSession getSession() {
+    @Nullable
+    public LocalSession getSession() {
         return session;
     }
 
@@ -111,7 +125,8 @@ public final class Request {
      *
      * @return the edit session, which may be null
      */
-    public @Nullable EditSession getEditSession() {
+    @Nullable
+    public EditSession getEditSession() {
         return editSession;
     }