Update the javadoc for some entries in WifiLinkLayerStats

Update the javadoc for contention_time_stats and tx_time_per_level to improve clarity.

Bug: 189123889

Test: atest com.android.server.wifi

Signed-off-by: Mingguang Xu <mingguangxu@google.com>
Change-Id: I9c8b76b8813aa48862b22d156731f6d6745f2840
2 files changed