Spec that addWrites advances oldestVersion minimally

The old wording only gave the lower bound oldestVersion >= newestVersion -
two billion, which an implementation could satisfy by discarding more
readable versions than necessary. State that oldestVersion becomes the
larger of its previous value and newestVersion - two billion.

include/ConflictSet.h

@@ -84,11 +84,10 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
    * `WriteRange` (see its field documentation); `count >= 0`; `writeVersion >=
    * newestVersion`.
    * @post Writes (writes[i], writeVersion) are recorded for every i < count,
-   * and `newestVersion` becomes `writeVersion`. `oldestVersion` advances as
-   * needed so that `oldestVersion >= newestVersion - two billion`;
-   * `oldestVersion <= newestVersion` is preserved. Calling with `count == 0`
-   * adds no write; `newestVersion` (and possibly `oldestVersion`) still
-   * advance. */
+   * and `newestVersion` becomes `writeVersion`. `oldestVersion` advances only
+   * as far as necessary: it becomes the larger of its previous value and
+   * `newestVersion - two billion`. Calling with `count == 0` adds no write;
+   * `newestVersion` (and possibly `oldestVersion`) still advance. */
   void addWrites(const WriteRange *writes, int count, int64_t writeVersion);
 
   /** Reads where readVersion < `oldestVersion` will result in `TooOld`.
@@ -231,11 +230,10 @@ void ConflictSet_check(const ConflictSet *cs,
  * `ConflictSet_WriteRange` (see its field documentation); `count >= 0`;
  * `writeVersion >= newestVersion`.
  * @post Writes (writes[i], writeVersion) are recorded for every i < count,
- * and `newestVersion` becomes `writeVersion`. `oldestVersion` advances as
- * needed so that `oldestVersion >= newestVersion - two billion`;
- * `oldestVersion <= newestVersion` is preserved. Calling with `count == 0`
- * adds no write; `newestVersion` (and possibly `oldestVersion`) still
- * advance. */
+ * and `newestVersion` becomes `writeVersion`. `oldestVersion` advances only
+ * as far as necessary: it becomes the larger of its previous value and
+ * `newestVersion - two billion`. Calling with `count == 0` adds no write;
+ * `newestVersion` (and possibly `oldestVersion`) still advance. */
 void ConflictSet_addWrites(ConflictSet *cs,
                            const ConflictSet_WriteRange *writes, int count,
                            int64_t writeVersion);