Currently the help output of `docker network ls --filter` is:
```
Options:
-f, --filter value Provide filter values (i.e. 'dangling=true') (default [])
...
```
This caused confusion as only the following filters are supported at the moment:
- `driver`
- `type`
- `name`
- `id`
- `label`
This fix update the help output of `docker network ls --filter` and `network_ls.md`.
The `dangling=true` description has been replace to:
```
Options:
-f, --filter filter Provide filter values (i.e. 'driver=bridge')
...
```
This fix fixes 28786.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Yong Tang