Document type2identifier function

In order to specify the properties which are expected to hold for this
function.

Author: thomasspriggs

src/ansi-c/type2name.h

@@ -20,6 +20,15 @@ class namespacet;
 
 std::string type2name(const typet &type, const namespacet &ns);
 
+/**
+ * Constructs a string describing the given type, which can be used as part of
+ * a `C` identifier. The resulting identifier is not guaranteed to uniquely
+ * identify the type in all cases.
+ * @param type Internal representation of the type to describe.
+ * @param ns Namespace for looking up any types on which the `type` parameter
+ *   depends.
+ * @return An identifying string which can be used as part of a `C` identifier.
+ */
 std::string type2identifier(const typet &type, const namespacet &ns);
 
 #endif // CPROVER_ANSI_C_TYPE2NAME_H