route: update package docs
route: update package docs

It's confirmed that this package works on FreeBSD 12-STABLE kernels.

Change-Id: I1f304d3885a351c8c9332b18d6842d1d8d6a66da
Reviewed-on: https://go-review.googlesource.com/c/157277
Run-TryBot: Mikio Hara <7d2a5fb8aa07e3795b8006c20159dd02aea6b94f@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net