Improve docker image rm reference docs

Copies the improved description from the man page
to the online reference docs.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 89bc5fbbae)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/rmi.md

@@ -26,6 +26,17 @@ Options:
       --no-prune   Do not delete untagged parents
 ```
 
+## Description
+
+Removes (and un-tags) one or more images from the host node. If an image has
+multiple tags, using this command with the tag as a parameter only removes the
+tag. If the tag is the only one for the image, both the image and the tag are
+removed.
+
+This does not remove images from a registry. You cannot remove an image of a
+running container unless you use the `-f` option. To see all images on a host
+use the [`docker image ls`](images.md) command.
+
 ## Examples
 
 You can remove an image using its short or long ID, its tag, or its digest. If