Update javadoc

Signed-off-by: Tran Ngoc Nhan <ngocnhan.tran1996@gmail.com>

core/src/main/java/org/springframework/security/access/expression/SecurityExpressionRoot.java

@@ -208,7 +208,8 @@ public abstract class SecurityExpressionRoot<T extends @Nullable Object> impleme
 	/**
 	 * Convenience method to access {@link Authentication#getPrincipal()} from
 	 * {@link #getAuthentication()}
-	 * @return
+	 * @return the <code>Principal</code> being authenticated or the authenticated
+	 * principal after authentication.
 	 */
 	public @Nullable Object getPrincipal() {
 		return getAuthentication().getPrincipal();