net: add documentation for skb recycling

net: add documentation for skb recycling

Commit 04a4bb55bcf35b63d40fd2725e58599ff8310dd7 ("net: add
skb_recycle_check() to enable netdriver skb recycling") added a
method for network drivers to recycle skbuffs, but while use of
this mechanism was documented in the commit message, it should
really have been added as a docbook comment as well -- this
patch does that.

Signed-off-by: Stephen Hemminger <[email protected]>
Signed-off-by: Lennert Buytenhek <[email protected]>
Signed-off-by: David S. Miller <[email protected]>