Add additional xref

Author: mborland

doc/modules/ROOT/nav.adoc

@@ -21,6 +21,7 @@
 *** xref:api_reference.adoc#api_iostream[`<iostream>`]
 *** xref:api_reference.adoc#api_ios[`<ios>`]
 *** xref:api_reference.adoc#api_numeric[`<numeric>`]
+*** xref:api_reference.adoc#api_string[`<string>`]
 ** xref:api_reference.adoc#api_macros[Macros]
 *** xref:api_reference.adoc#api_macro_literals[Literals]
 *** xref:api_reference.adoc#api_macro_configuration[Configuration]