Squiz/SelfMemberReference: update XML doc (#1108)

- Improve sniff description explicitly mentioning where the spaces are
checked and adding context as to when the `self` keyword is verified.
- Fix the code examples for checking `self` case and spaces around the
double colon to ensure they trigger the sniff (the sniff checks if the
call is made within a class).
- Improve the titles of the code examples related to spacing around
double colons.
- Remove unnecessary methods from the code examples related to using
`self` instead of the class name when referencing a static member.

Author: rodrigoprimo

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

@@ -1,59 +1,77 @@
 <documentation title="Self Member Reference">
     <standard>
     <![CDATA[
-    The self keyword should be used instead of the current class name, should be lowercase, and should not have spaces before or after it.
+    The self keyword must be lowercase.
     ]]>
     </standard>
     <code_comparison>
         <code title="Valid: Lowercase self used.">
         <![CDATA[
-<em>self</em>::foo();
+class Bar {
+    public function baz() {
+        <em>self</em>::foo();
+    }
+}
         ]]>
         </code>
         <code title="Invalid: Uppercase self used.">
         <![CDATA[
-<em>SELF</em>::foo();
+class Bar {
+    public function baz() {
+        <em>SELF</em>::foo();
+    }
+}
         ]]>
         </code>
     </code_comparison>
+    <standard>
+    <![CDATA[
+    There must be no spaces before or after the double colon operator.
+    ]]>
+    </standard>
     <code_comparison>
-        <code title="Valid: Correct spacing used.">
+        <code title="Valid: No spaces around the double colon operator.">
         <![CDATA[
-self<em></em>::<em></em>foo();
+class Bar {
+    public function baz() {
+        self<em></em>::<em></em>foo();
+    }
+}
         ]]>
         </code>
-        <code title="Invalid: Incorrect spacing used.">
+        <code title="Invalid: Spaces around double colon operator.">
         <![CDATA[
-self<em> </em>::<em> </em>foo();
+class Bar {
+    public function baz() {
+        self<em> </em>::<em> </em>foo();
+    }
+}
         ]]>
         </code>
     </code_comparison>
+    <standard>
+    <![CDATA[
+    The self keyword must be used instead of the current class name.
+    ]]>
+    </standard>
     <code_comparison>
         <code title="Valid: Self used as reference.">
         <![CDATA[
-class Foo
-{
-    public static function bar()
-    {
-    }
+class Foo {
+    public function bar() {}
 
-    public static function baz()
-    {
+    public function baz() {
         <em>self</em>::bar();
     }
 }
         ]]>
         </code>
         <code title="Invalid: Local class name used as reference.">
         <![CDATA[
-class <em>Foo</em>
-{
-    public static function bar()
-    {
-    }
+class <em>Foo</em> {
+    public function bar() {}
 
-    public static function baz()
-    {
+    public function baz() {
         <em>Foo</em>::bar();
     }
 }