Python: Add explanatory QLDoc comment

Author: tausbn

python/ql/src/semmle/python/dataflow/new/SensitiveDataSources.qll

@@ -194,6 +194,12 @@ private module SensitiveDataModeling {
     not result.regexpMatch(notSensitiveRegexp())
   }
 
+  /**
+   * Returns strings (primarily the names of various program entities) that may contain sensitive data
+   * with the classification `classification`.
+   *
+   * This is a helper predicate, used to limit the number of regexp matches that have to be performed.
+   */
   pragma[nomagic]
   private string sensitiveString(SensitiveDataClassification classification) {
     result in [