Clarify PascalCase class naming requirement

Added a friendlier description of the PascalCase requirements.

Adds an extra example to clarify that it's not just a leading capital
we're looking for - we also need only letters and numbers.

src/Standards/Squiz/Docs/Classes/ValidClassNameStandard.xml

@@ -1,11 +1,11 @@
 <documentation title="Valid Class Name">
     <standard>
     <![CDATA[
-    Class names must be written in Pascal case.
+    Class names must be written in Pascal case. This means that it starts with a capital letter, and the first letter of each word in the class name is capitalized. Only letters and numbers are allowed.
     ]]>
     </standard>
     <code_comparison>
-        <code title="Valid: Class name is written in Pascal case.">
+        <code title="Valid: Class name starts with a capital letter.">
         <![CDATA[
 class <em>PascalCaseStandard</em>
 {
@@ -19,5 +19,21 @@ class <em>notPascalCaseStandard</em>
 }
         ]]>
         </code>
-        </code_comparison>
+    </code_comparison>
+    <code_comparison>
+        <code title="Valid: Class name contains only letters and numbers.">
+        <![CDATA[
+class <em>PSR7Response</em>
+{
+}
+        ]]>
+        </code>
+        <code title="Invalid: Class name contains underscores.">
+        <![CDATA[
+class <em>PSR7_Response</em>
+{
+}
+        ]]>
+        </code>
+    </code_comparison>
 </documentation>