Simplify swappiness check

As suggested in https://github.com/docker/docker/pull/14004/files#r34022527

The concern there is we can't differentiate whether user explicitly
asked for an invalid value of -1 or he did not specify anything.

I don't think this would be a problem, because:
 - like all other default values like zero, we can't differentiate
   user specify it or not, most of which, zeros are also invalid, so
   default is default, we show these default values in help info,
   so users would know if they set value as default, it'll be like
   they set nothing.
 - we can't do this kind of string check in REST api request, so
   it'll make the behave different from docker command and RESTapi.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Upstream-commit: 7e4cf8a7ec
Component: cli

components/cli/docs/reference/run.md

@@ -617,7 +617,8 @@ and require killing system processes to free memory.
 By default, a container's kernel can swap out a percentage of anonymous pages.
 To set this percentage for a container, specify a `--memory-swappiness` value
 between 0 and 100. A value of 0 turns off anonymous page swapping. A value of
-100 sets all anonymous pages as swappable.
+100 sets all anonymous pages as swappable. By default, if you are not using
+`--memory-swappiness`, memory swappiness value will be inherited from the parent.
 
 For example, you can set: