gobject: Cross-link from GType reference docs to GType conventions page

Make it a little easier to find the GType conventions page, which I
guess should be the canonical guide to how to name things.

This adds a brief mention of the valid characters in a type name to the
conventions page.

https://bugzilla.gnome.org/show_bug.cgi?id=743018

docs/reference/gobject/tut_gtype.xml

@@ -232,6 +232,10 @@ struct _GTypeValueTable
         There are a number of conventions users are expected to follow when creating new types
         which are to be exported in a header file:
         <itemizedlist>
+          <listitem><para>
+            Type names (including object names) must be at least three
+            characters long and start with ‘a–z’, ‘A–Z’ or ‘_’.
+          </para></listitem>
           <listitem><para>
             Use the <function>object_method</function> pattern for function names: to invoke
             the method named foo on an instance of object type bar, call 

gobject/gtype.c

@@ -76,10 +76,10 @@
  * separately (typically by using #GArray or #GPtrArray) and put a pointer
  * to the buffer in the structure.
  *
- * A final word about type names: Such an identifier needs to be at least
- * three characters long. There is no upper length limit. The first character
- * needs to be a letter (a-z or A-Z) or an underscore '_'. Subsequent
- * characters can be letters, numbers or any of '-_+'.
+ * As mentioned in the [GType conventions][gtype-conventions], type names must
+ * be at least three characters long. There is no upper length limit. The first
+ * character must be a letter (a–z or A–Z) or an underscore (‘_’). Subsequent
+ * characters can be letters, numbers or any of ‘-_+’.
  */