iflib: document new sysctls

Document the new tx related sysctls I've added to iflib in the
last few months: tx_reclaim_ticks & tx_reclaim_thresh

Sponsored by: Netflix
Reviewed by: ziaee
Differential Revision: https://reviews.freebsd.org/D54564
MFC after: 3 days

(cherry picked from commit 83e6f0d7807b4ebc74bc58e6f03adde56db942bf)