Add comment on min-partition-size

Change-Id: Ia272fc58c35ccfb164e1b8743634f55e226f4d74
diff --git a/apps/aomenc.c b/apps/aomenc.c
index 5e4522d..3594162 100644
--- a/apps/aomenc.c
+++ b/apps/aomenc.c
@@ -451,7 +451,9 @@
 static const arg_def_t min_partition_size =
     ARG_DEF(NULL, "min-partition-size", 4,
             "Set min partition size "
-            "(4:4x4, 8:8x8, 16:16x16, 32:32x32, 64:64x64, 128:128x128)");
+            "(4:4x4, 8:8x8, 16:16x16, 32:32x32, 64:64x64, 128:128x128)."
+            "On frame with 4k+ resolutions or higher speed settings, the min "
+            "partition size will have a minimum of 8.");
 static const arg_def_t max_partition_size =
     ARG_DEF(NULL, "max-partition-size", 128,
             "Set max partition size "