Update Checkstyle checks

Lets update checkstyle rules as follows, and update the code to match.

New rules:
* To match SE-EDU Java Coding Standard (basic):
  1. DefaultComesLast
  2. NoWhitespaceBeforeCaseDefaultColon
  3. AtclauseOrder
  4. JavadocBlockTagLocation
  5. JavadocContentLocationCheck
  6. JavadocMissingLeadingAsterisk
  7. JavadocMissingWhitespaceAfterAsterisk
* To match SE-EDU Java Coding Standard (intermediate):
  1. MethodParamPad
  2. MissingJavadocType
* To match Google's Java Code Style:
  1. JavadocTagContinuationIndentation

Updates to existing rules:
1. Update default throwsIndent value for the Indentation
   from 4 (incorrect) to 8 (correct).

Updates to code to match the above:
1. HelpWindow file: to match JavadocTagContinuationIndentation

config/checkstyle/checkstyle.xml

@@ -245,6 +245,9 @@
     -->
     <module name ="DeclarationOrder"/>
 
+    <!-- Checks that default is after all cases in a switch statement -->
+    <module name="DefaultComesLast"/>
+
     <module name="ModifierOrder">
       <!-- Warn if modifier order is inconsistent with JLS3 8.1.1, 8.3.1, and
            8.4.3.  The prescribed order is:
@@ -340,6 +343,7 @@
 
     <module name="Indentation">
       <property name="caseIndent" value="0" />
+      <property name="throwsIndent" value="8" />
     </module>
 
     <module name="NoWhitespaceBefore">
@@ -348,6 +352,11 @@
       <property name="allowLineBreaks" value="true"/>
     </module>
 
+    <module name="NoWhitespaceBeforeCaseDefaultColon"/>
+
+    <!-- Checks that there is no whitespace between method/constructor name and open parenthesis. -->
+    <module name="MethodParamPad"/>
+
     <module name="ParenPad">
       <!-- Checks that there is no whitespace before close parenthesis or after open parenthesis. -->
       <property name="severity" value="warning"/>
@@ -376,6 +385,24 @@
     JAVADOC CHECKS
     -->
 
+    <!-- Checks that all block-tags are ordered correctly. -->
+    <module name="AtclauseOrder"/>
+
+    <!-- Checks that Javadoc block tags appear only at the beginning of the line. -->
+    <module name="JavadocBlockTagLocation"/>
+
+    <!-- Checks that all Javadoc comments start from the second line. -->
+    <module name="JavadocContentLocationCheck" />
+
+    <!-- Checks that each line in Javadoc has leading asterisks. -->
+    <module name="JavadocMissingLeadingAsterisk"/>
+
+    <!-- Checks that each non-empty line in Javadoc has whitespace after leading asterisk. -->
+    <module name="JavadocMissingWhitespaceAfterAsterisk"/>
+
+    <!-- Checks that for block tags, indentation of continuation lines is at least 4 spaces. -->
+    <module name="JavadocTagContinuationIndentation"/>
+
     <!-- Checks the Javadoc's format for every class, enumeration and interface. -->
     <module name="JavadocType">
       <property name="allowMissingParamTags" value="true"/>
@@ -400,5 +427,8 @@
       <property name="ignoreMethodNamesRegex" value="(set.*|get.*)"/>
     </module>
 
+    <!-- Checks that every public class, enumeration and interface has a header comment. -->
+    <module name="MissingJavadocType"/>
+
   </module>
 </module>

src/main/java/seedu/address/ui/HelpWindow.java

@@ -47,20 +47,20 @@ public HelpWindow() {
     /**
      * Shows the help window.
      * @throws IllegalStateException
-     * <ul>
-     *     <li>
-     *         if this method is called on a thread other than the JavaFX Application Thread.
-     *     </li>
-     *     <li>
-     *         if this method is called during animation or layout processing.
-     *     </li>
-     *     <li>
-     *         if this method is called on the primary stage.
-     *     </li>
-     *     <li>
-     *         if {@code dialogStage} is already showing.
-     *     </li>
-     * </ul>
+     *     <ul>
+     *         <li>
+     *             if this method is called on a thread other than the JavaFX Application Thread.
+     *         </li>
+     *         <li>
+     *             if this method is called during animation or layout processing.
+     *         </li>
+     *         <li>
+     *             if this method is called on the primary stage.
+     *         </li>
+     *         <li>
+     *             if {@code dialogStage} is already showing.
+     *         </li>
+     *     </ul>
      */
     public void show() {
         logger.fine("Showing help page about the application.");