Improve type name headings in the nav

Author: mborland

doc/modules/ROOT/nav.adoc

@@ -16,11 +16,11 @@
 ** xref:api_reference.adoc#api_ref_enums[Enums]
 ** xref:api_reference.adoc#api_ref_constants[Constants]
 ** xref:api_reference.adoc#api_ref_macros[Macros]
-* xref:generic_decimal.adoc[]
+* xref:generic_decimal.adoc[IEEE 754 Decimal Types]
 ** xref:decimal32_t.adoc[]
 ** xref:decimal64_t.adoc[]
 ** xref:decimal128_t.adoc[]
-* xref:fast_types.adoc[]
+* xref:fast_types.adoc[Non-IEEE 754 Decimal Types]
 ** xref:decimal_fast32_t.adoc[]
 ** xref:decimal_fast64_t.adoc[]
 ** xref:decimal_fast128_t.adoc[]