Add QLDoc explaining reverseRead

shared/dataflow/codeql/dataflow/internal/DataFlowImplConsistency.qll

@@ -244,6 +244,13 @@ module MakeConsistency<
 
   private predicate hasPost(Node n) { exists(PostUpdateNode post | post.getPreUpdateNode() = n) }
 
+  /**
+   * Consider code like `a.b.f = source()`. There is flow from `source()` to
+   * `[post] a.b` (with an appropriate access path), but we also want there to
+   * be flow to `[post] a` (with an appropriate access path). The data flow
+   * library is able to infer this step because there is a read step from `a`
+   * to `a.b`, as long as the post-update node for `a` exists.
+   */
   query predicate reverseRead(Node n, string msg) {
     exists(Node n2 | readStep(n, _, n2) and hasPost(n2) and not hasPost(n)) and
     not Input::reverseReadExclude(n) and