Python: Add missing QLDoc

Author: yoff

python/ql/src/semmle/python/Concepts.qll

@@ -308,6 +308,13 @@ class ErrorInfoSource extends DataFlow::Node {
 
 /** Provides a class for modeling new sources of error information, say via APIs. */
 module ErrorInfoSource {
+  /**
+   * A data-flow node that carries information about an error. Such information should
+   * rarely be exposed directly to the user.
+   *
+   * Extend this class to model new APIs. If you want to refine existing API models,
+   * extend `ErrorInfoSource` instead.
+   */
   abstract class Range extends DataFlow::Node { }
 }
 
@@ -325,6 +332,12 @@ class ExceptionSource extends ErrorInfoSource::Range {
 
 /** Provides a class for modeling new sources of exceptions, say via APIs. */
 module ExceptionSource {
+  /**
+   * A data-flow node that represents the creation or introduction of an exception.
+   *
+   * Extend this class to model new APIs. If you want to refine existing API models,
+   * extend `ExceptionSource` instead.
+   */
   abstract class Range extends DataFlow::Node { }
 }