doc: nrf: known_issues: Add known issue for download_client and TLS

Add known issue for download_client with TLS on nRF91 series devices.
The download_client is deprecated since NCS 3.0.0 and replaced
by the downloader.

Signed-off-by: Eivind Jølsgard <[email protected]>

doc/nrf/releases_and_maturity/known_issues.rst

@@ -3706,6 +3706,22 @@ NCSDK-6238: Socket API calls might hang when using Download client
 
   **Workaround:** Set :kconfig:option:`CONFIG_DOWNLOAD_CLIENT_RANGE_REQUESTS` with the :ref:`lib_download_client` library.
 
+.. rst-class:: v2-9-0-nRF54H20-rc1 v2-9-0 v2-8-0 v2-7-0 v2-6-2 v2-6-1 v2-6-0 v2-5-3 v2-5-2 v2-5-1 v2-5-0 v2-4-4 v2-4-3 v2-4-2 v2-4-1 v2-4-0 v2-3-0 v2-2-0 v2-1-4 v2-1-3 v2-1-2 v2-1-1 v2-1-0 v2-0-2 v2-0-1 v2-0-0 v1-9-2 v1-9-1 v1-9-0 v1-8-0 v1-7-1 v1-7-0 v1-6-1 v1-6-0 v1-5-2 v1-5-1 v1-5-0 v1-4-2 v1-4-1 v1-4-0
+
+NCSDK-31645: Download client might fail with error ``-EMSGSIZE`` when using TLS on nRF91 Series devices
+  The nRF91 Series modem has a size limit of 2303 bytes for receiving TLS packages.
+  This limitation results in the download client failing with error ``-EMSGSIZE`` when the HTTP package responded by the server is larger than the modem size limit.
+  The HTTP package includes both the HTTP header and payload.
+  Samples and applications that are affected include those that use the :ref:`lib_download_client` library to download files through HTTP, or those that use :ref:`lib_fota_download` with modem updates enabled.
+
+  **Affected platforms:** nRF9160 nRF9131 nRF9151 nRF9161
+
+  **Workaround:** Set the :c:member:`download_client_cfg.frag_size_override` in the download client configuration.
+  This enables HTTP range requests with a limited payload size as set in the :c:member:`download_client_cfg.frag_size_override` configuration.
+  The fragment size must be set so that the total size of the HTTP header plus the payload must not exceed the modem limit of 2303 bytes.
+  The HTTP header size is dependent on the server in use.
+  If you are experiencing this issue on a deployed product, reducing the HTTP header size responded by the server can also resolve this issue.
+
 .. rst-class:: v1-1-0
 
 Jobs not received after reset