doc: tls: socket needs to be established to enable ulp

To enable TLS ulp socket needs to be in established state.
This was added in commit d91c3e17 ("net/tls: Only attach
to sockets in ESTABLISHED state"), in 2018.

Signed-off-by: Ulrich Weber <ulrich.weber@gmail.com>
Link: https://patch.msgid.link/20250626145618.15464-1-ulrich.weber@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>