Add description about Tx scheduling timestamp upper limit.
If timestamp exceeds the value, it is marked by PMD as being
into "too-distant-future" and not scheduled at all
(is being sent without any wait).
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
b5f615612a