doc: Add some missing comments about android default route (#828)

src/hp/interfaces/linux.rs

@@ -63,6 +63,10 @@ async fn default_route_proc() -> Result<Option<DefaultRouteDetails>> {
     Ok(None)
 }
 
+/// Try find the default route by parsing the "ip route" command output.
+///
+/// We use this on Android where /proc/net/route can be missing entries or have locked-down
+/// permissions.  See also comments in /~https://github.com/tailscale/tailscale/pull/666.
 #[cfg(target_os = "android")]
 pub async fn default_route_android_ip_route() -> Result<Option<DefaultRouteDetails>> {
     use tokio::process::Command;