MINOR: session: document explicitely that session_add_conn() is safe

MINOR: session: document explicitely that session_add_conn() is safe

A set of recent patches have simplified management of backend connection
attached to sessions. The API is now stricter to prevent any misuse.

One of this change is the addition of a BUG_ON() in session_add_conn(),
which ensures that a connection is not attached to a session if its
<owner> field points to another entry.

On older haproxy releases, this assertion could not be enforced due to
NTLM as a connection is turned as private during its transfer. When
using a true multiplexed protocol on the backend side, the connection
could be assigned in turn to several sessions. However, NTLM is now only
applied for HTTP/1.1 as it does not make sense if the connection is
already shared.

To better clarify this situation, extend the comment on BUG_ON() inside
session_add_conn().