man: tc-fq_codel: add drop_batch

Let's describe the drop_batch parameter added to tc command
by Commit 7868f802e2d9 ("tc: fq_codel: add drop_batch parameter")

Signed-off-by: Yuki Inoguchi <inoguchi.yuki@fujitsu.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>