C++: add comments in new iterator models

cpp/ql/src/semmle/code/cpp/models/implementations/Iterator.qll

@@ -338,6 +338,10 @@ class BeginOrEndFunction extends MemberFunction, TaintFunction, GetIteratorFunct
   }
 }
 
+/**
+ * The `std::front_inserter`, `std::inserter`, and `std::back_inserter`
+ * functions.
+ */
 class InserterIteratorFunction extends GetIteratorFunction {
   InserterIteratorFunction() {
     this.hasName(["front_inserter", "inserter", "back_inserter"]) and

cpp/ql/src/semmle/code/cpp/models/interfaces/Iterator.qll

@@ -16,6 +16,13 @@ import semmle.code.cpp.models.Models
  */
 abstract class IteratorReferenceFunction extends Function { }
 
+/**
+ * A function which takes a container and returns an iterator over that container.
+ */
 abstract class GetIteratorFunction extends Function {
+  /**
+   * Holds if the return value or buffer represented by `output` is an iterator over the container
+   * passd in the argument, qualifier, or buffer represented by `input`.
+   */
   abstract predicate getsIterator(FunctionInput input, FunctionOutput output);
 }