C++: Add file-level QLDoc.

cpp/ql/lib/semmle/code/cpp/models/interfaces/PartialFlow.qll

@@ -1,3 +1,16 @@
+/**
+ * Provides an abstract class to override the implicit assumption that a
+ * dataflow/taint-tracking model always fully override the parameters they are
+ * are modelled as writing to. To use this QL library, create a QL class
+ * extending `PartialFlowFunction` with a characteristic predicate that selects
+ * the function or set of functions you are modeling and override the
+ * `isPartialWrite` predicate.
+ *
+ * Note: Since both `DataFlowFunction` and `TaintFunction` extend this class
+ * you don't need to explicitly add this as a base class if your QL class
+ * already extends either `DataFlowFunction` or `TaintFunction`.
+ */
+
 import semmle.code.cpp.Function
 import FunctionInputsAndOutputs
 import semmle.code.cpp.models.Models