luci-mod-network: improve description for filterwin2k option again

Stop using the word useless both in the name and the description and call
the feature what it actually is (Filter SRV/SOA service discovery).

Signed-off-by: Lukas Tribus <lukas@ltri.eu>
[slightly reword commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>