Clarify that device_one_time_keys_count is only optional if no unclaimed one-time keys exist

Fixes: #1057
Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

changelogs/client_server/newsfragments/2245.clarification

@@ -0,0 +1 @@
+`device_one_time_keys_count` is only optional if no unclaimed one-time keys exist.

content/client-server-api/modules/end_to_end_encryption.md

@@ -1778,7 +1778,7 @@ a way to identify the server's support for fallback keys.
 | Parameter                        | Type              | Description                                                                                                            |
 |----------------------------------|-------------------|------------------------------------------------------------------------------------------------------------------------|
 | device_lists                     | DeviceLists       | Optional. Information on e2e device updates. Note: only present on an incremental sync.                                |
-| device_one_time_keys_count       | {string: integer} | Optional. For each key algorithm, the number of unclaimed one-time keys currently held on the server for this device. If an algorithm is unlisted, the count for that algorithm is assumed to be zero.  If this entire parameter is missing, the count for all algorithms is assumed to be zero. |
+| device_one_time_keys_count       | {string: integer} | **Required if unclaimed one-time keys exist.** For each key algorithm, the number of unclaimed one-time keys currently held on the server for this device. If the count for an algorithm is zero, servers MAY omit that algorithm. If the count for all algorithms is zero, servers MAY omit this parameter entirely. |
 | device_unused_fallback_key_types | [string]          | **Required.** The unused fallback key algorithms.                                                                      |
 
 `DeviceLists`