C++: Add a definition of taint to the models library.

Author: geoffw0

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

@@ -14,6 +14,9 @@ import semmle.code.cpp.models.Models
 /**
  * A library function for which a taint-tracking library should propagate taint
  * from a parameter or qualifier to an output buffer, return value, or qualifier.
+ *
+ * An expression is tainted if it could be influenced by an attacker to have
+ * an unusual value.
  * 
  * Note that this does not include direct copying of values; that is covered by
  * DataFlowModel.qll