doc:proxy-setting info added in pull cmd.

Replace Note with a new secion, reduce characters in
per line in 80. Add statement suggested by
https://github.com/thaJeztah

Signed-off-by: Lin Lu <doraalin@163.com>

docs/reference/commandline/pull.md

@@ -27,6 +27,15 @@ can `pull` and try without needing to define and configure your own.
 To download a particular image, or set of images (i.e., a repository),
 use `docker pull`.
 
+## Proxy configuration
+
+If you are behind a HTTP proxy server, for example in corporate settings,
+before open a connect to registry, you may need to configure the Docker
+daemon's proxy settings, using the `HTTP_PROXY`, `HTTPS_PROXY`, and `NO_PROXY`
+environment variables. To set these environment variables on a host using
+`systemd`, refer to the [control and configure Docker with systemd](../../admin/systemd.md#http-proxy)
+for variables configuration.
+
 ## Examples
 
 ### Pull an image from Docker Hub