Add reference to supported data types section (#1893)

docs/api-reference/library-specification.rst

@@ -109,4 +109,6 @@ This section provides all the enumerations used.
 
 .. doxygenenum:: ncclRedOp_t
 
+.. _rccl-supported-data-types:
+
 .. doxygenenum:: ncclDataType_t