docs: syntax highlighting for the code examples

In the sections Concepts, Tools and Tutorial.

https://bugzilla.gnome.org/show_bug.cgi?id=736914

docs/reference/gobject/glib-mkenums.xml

@@ -50,13 +50,13 @@ in @ characters.
 Certain keywords enclosed in @ characters will be substituted in the
 emitted text. For the substitution examples of the keywords below,
 the following example enum definition is assumed:
-<programlisting>
+<informalexample><programlisting>
 typedef enum
 {
   PREFIX_THE_XVALUE    = 1 &lt;&lt; 3,
   PREFIX_ANOTHER_VALUE = 1 &lt;&lt; 4
 } PrefixTheXEnum;
-</programlisting>
+</programlisting></informalexample>
 <variablelist>
 <varlistentry>
 <term>@EnumName@</term>
@@ -185,7 +185,7 @@ Per value definition, the options "skip" and "nick" are supported.
 The former causes the value to be skipped, and the latter can be used to
 specify the otherwise auto-generated nickname.
 Examples:
-<programlisting>
+<informalexample><programlisting>
 typedef enum /*&lt; skip &gt;*/
 {
   PREFIX_FOO
@@ -197,7 +197,7 @@ typedef enum /*&lt; flags,prefix=PREFIX &gt;*/
   PREFIX_THE_SECOND_VALUE,
   PREFIX_THE_THIRD_VALUE,	/*&lt; nick=the-last-value &gt;*/
 } PrefixTheFlagsEnum;
-</programlisting>
+</programlisting></informalexample>
 </para>
 </refsect2>
 </refsect1>
@@ -272,10 +272,10 @@ Template for auto-generated comments, the default (for C code generations) is
 <listitem><para>
 Read templates from the given file. The templates are enclosed in
 specially-formatted C comments
-<programlisting>
+<informalexample><programlisting>
 /*** BEGIN section ***/
 /*** END section ***/
-</programlisting>
+</programlisting></informalexample>
 where section may be <literal>file-header</literal>,
 <literal>file-production</literal>, <literal>file-tail</literal>,
 <literal>enumeration-production</literal>, <literal>value-header</literal>,