C++: add some missing QLDoc

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

@@ -1,3 +1,10 @@
+/**
+ * Provides implementation classes modeling C++ iterators, including
+ * `std::iterator`, `std::iterator_traits`, and types meeting the
+ * `LegacyIterator` named requirement. See `semmle.code.cpp.models.Models` for
+ * usage information.
+ */
+
 import cpp
 import semmle.code.cpp.models.interfaces.Taint
 import semmle.code.cpp.models.interfaces.DataFlow
@@ -187,6 +194,9 @@ class IteratorAssignArithmeticOperator extends Operator, DataFlowFunction, Taint
   }
 }
 
+/**
+ * A non-member `operator[]` function for an iterator type.
+ */
 class IteratorArrayOperator extends Operator, TaintFunction {
   IteratorArrayOperator() {
     this.hasName("operator[]") and

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

@@ -1,3 +1,9 @@
+/**
+ * Provides implementation classes modeling `std::string` and other
+ * instantiations of`std::basic_string`. See `semmle.code.cpp.models.Models`
+ * for usage information.
+ */
+
 import semmle.code.cpp.models.interfaces.Taint
 import semmle.code.cpp.models.implementations.Iterator
 
@@ -66,6 +72,9 @@ class StdStringAppend extends TaintFunction {
     getParameter(result).getType() = getDeclaringType().getTemplateArgument(0) // i.e. `std::basic_string::CharT`
   }
 
+  /**
+   * Gets the index of a parameter to this function that is an iterator.
+   */
   int getAnIteratorParameter() { getParameter(result).getType() instanceof Iterator }
 
   override predicate hasTaintFlow(FunctionInput input, FunctionOutput output) {