C++: Add missing QLDoc.

Author: criemen

cpp/ql/src/Diagnostics/FailedExtractions.qll

@@ -21,23 +21,30 @@ class ReportableError extends Diagnostic {
   }
 }
 
-newtype TExtractionError =
+private newtype TExtractionError =
   TReportableError(ReportableError err) or
   TCompilationFailed(Compilation c, File f) {
     f = c.getAFileCompiled() and not c.normalTermination()
   } or
   // Report generic extractor errors only if we haven't seen any other error-level diagnostic
   TUnknownError(CompilerError err) { not exists(ReportableError e) }
 
+/**
+ * Superclass for the extraction error hierarchy.
+ */
 class ExtractionError extends TExtractionError {
   string toString() { none() }
 
+  /** Gets the error message for this error. */
   string getErrorMessage() { none() }
 
+  /** Gets the file this error occured in. */
   File getFile() { none() }
 
+  /** Gets the location this error occured in. */
   Location getLocation() { none() }
 
+  /** Gets the SARIF severity of this error. */
   int getSeverity() {
     // Unfortunately, we can't distinguish between errors and fatal errors in SARIF,
     // so all errors have severity 2.