Documentation: TLS: fix stat counters description

Add missing description of counters.
Split tx_tls_encrypted counter into two, to give packets
and bytes indications.

Fixes: f42c104f

 ("Documentation: add TLS offload documentation")
Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>