doc: tls_credentials_shell: Document credential load functionality

Document the credential load functionality added to `cred buf` command.

Signed-off-by: Joakim Andersson <joerchan@gmail.com>

Author: joerchan

doc/connectivity/networking/api/tls_credentials_shell.rst

@@ -15,7 +15,11 @@ Buffer Credential (``buf``)
 
 Buffer data incrementally into the credential buffer so that it can be added using the :ref:`tls_credentials_shell_add_cred` command.
 
-Alternatively, clear the credential buffer.
+Alternatively:
+
+   - Clear the credential buffer.
+
+   - Load credential directly to the credential buffer, ending with ``Ctrl + c``.
 
 Usage
 -----
@@ -28,6 +32,14 @@ To append ``<DATA>`` to the credential buffer, use:
 
 Use this as many times as needed to load the full credential into the credential buffer, then use the :ref:`tls_credentials_shell_add_cred` command to store it.
 
+To load ``<DATA>`` directly to the credential buffer, use:
+
+.. code-block:: shell
+
+   cred buf load
+   <DATA>
+   Ctrl + c
+
 To clear the credential buffer, use:
 
 .. code-block:: shell