man: tc-taprio.8: document the full offload feature

man: tc-taprio.8: document the full offload feature

Since this feature's introduction in commit 9c66d1564676 ("taprio: Add
support for hardware offloading") from kernel v5.4, it never got
documented in the man pages. Due to this reason, we see customer reports
of seemingly contradictory information: the community manpages claim
there is no support for full offload, nonetheless many silicon vendors
have already implemented it.

This patch documents the full offload feature (enabled by specifying
"flags 2" to the taprio qdisc) and gives one more example that tries to
illustrate some of the finer points related to the usage.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>