[Docs] Add XML doc for Squiz.Classes.ValidClassName sniff (#842)

* Add XML doc for Squiz\Classes\ValidClassName sniff

* [Doc] Use em tags for ValidClassName doc XML

Uses `<em>` tags in the code sample for Squiz.Classes.ValidClassName to
highlight the critical part of the sample.

* 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.

Author: braindawg

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

@@ -0,0 +1,39 @@
+<documentation title="Valid Class Name">
+    <standard>
+    <![CDATA[
+    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 starts with a capital letter.">
+        <![CDATA[
+class <em>PascalCaseStandard</em>
+{
+}
+        ]]>
+        </code>
+        <code title="Invalid: Class name does not start with a capital letter.">
+        <![CDATA[
+class <em>notPascalCaseStandard</em>
+{
+}
+        ]]>
+        </code>
+    </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>