docs: tls: Enhance TLS resync async process documentation

docs: tls: Enhance TLS resync async process documentation

Expand the tls-offload.rst documentation to provide a more detailed
explanation of the asynchronous resync process, including the role
of struct tls_offload_resync_async in managing resync requests on
the kernel side.

Also, add documentation for helper functions
tls_offload_rx_resync_async_request_start/ _end/ _cancel.

Signed-off-by: Shahar Shitrit <shshitrit@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/1768298883-1602599-1-git-send-email-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>