Clarify the semantics of since (#8624)

For reference: https://github.com/pouchdb/pouchdb/blob/30fd84f1ae88ca8687c2842bd131e67e7cf7c41c/packages/node_modules/pouchdb-adapter-leveldb-core/src/index.js#L1063

Co-authored-by: Gareth Bowen <gareth@medic.org>
Co-authored-by: Alex Anderson <191496+alxndrsn@users.noreply.github.com>

Author: 3 people

docs/_includes/api/changes.html

@@ -19,7 +19,7 @@
   * `options.attachments`: Include attachments.
     - `options.binary`: Return attachment data as Blobs/Buffers, instead of as base64-encoded strings.
 * `options.descending`: Reverse the order of the output documents.
-* `options.since`: Start the results from the change immediately after the given sequence number. You can also pass `'now'` if you want only new changes (when `live` is `true`).
+* `options.since`: Start the results from the change immediately after the given sequence number. You can also pass `'now'` if you want only new changes (when `live` is `true`). Ignored if `descending` is true.
 * `options.limit`: Limit the number of results to this number.
 * `options.timeout`: Request timeout (in milliseconds), use `false` to disable.
 * `options.heartbeat`: For http adapter only, time in milliseconds for server to give a heartbeat to keep long connections open. Defaults to 10000 (10 seconds), use `false` to disable the default.