category-ads-all: add sigmob.cn

and update comments

data/category-ads

@@ -1,4 +1,5 @@
-# This file contains domains that clearly serving ads
+# This file contains domains that clearly serving for ads or tracking.
+# We can not seperate ads and tracking domains clearly, so they are mixed together.
 include:category-ads-ir
 
 include:36kr @ads
@@ -79,7 +80,6 @@ ads.huan.tv
 ads.huantest.com
 ads.trafficjunky.net
 ads.wteam.xyz
-adservice.sigmob.cn
 adtechus.com
 adtrue.com
 adxprtz.com