C++: Fix QLDoc.

Author: MathiasVP

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/SsaInternals.qll

@@ -769,7 +769,8 @@ private predicate adjustForPointerArith(PostUpdateNode pun, SourceVariable sv, I
  * `nodeFrom` is a definition or use of `sv` at index `i1` at basic
  * block `bb1`.
  *
- * `uncertain` is `true` if the this is an uncertain definition.
+ * `uncertain` is `true` if `(bb1, i1)` is a definition, and that definition
+ * is guaranteed to overwrite the entire allocation.
  */
 private predicate ssaFlowImpl(
   IRBlock bb1, int i1, SourceVariable sv, Node nodeFrom, Node nodeTo, boolean uncertain