[DOCS] Added multi fields link to mapping page.

docs/reference/mapping.asciidoc

@@ -67,6 +67,8 @@ include::mapping/fields.asciidoc[]
 
 include::mapping/types.asciidoc[]
 
+include::mapping/multi-fields.asciidoc[]
+
 include::mapping/date-format.asciidoc[]
 
 include::mapping/dynamic-mapping.asciidoc[]