Quadlet docs: add section on referencing other template Quadlets

Signed-off-by: cyqsimon <28627918+cyqsimon@users.noreply.github.com>

docs/source/markdown/podman-systemd.unit.5.md

@@ -204,6 +204,37 @@ create a drop-in file like `sleep@10.container.d/10-image.conf`:
 Image=quay.io/centos/centos
 ```
 
+#### Referencing other template Quadlets
+
+It often occurs that you may want to reference another template Quadlet
+from your Quadlet. For example, you may want each instance of `foo@.container`
+to mount a distinct volume. In this case you can create a template Quadlet
+`foo-data@.volume` (the name is arbitrary) and reference it in `foo@.container`:
+
+`foo-data@.volume`:
+```
+[Volume]
+# Optional: automatic naming does work with template Quadlets too.
+# This is just to demonstrate the behaviour of systemd specifiers (e.g. %i).
+VolumeName=my-name-%i
+```
+
+`foo@.container`:
+```
+[Container]
+Volume=foo-data@.volume:/mnt/path
+```
+
+Note that we reference the template name `foo-data@.volume` instead of
+the instance name `foo-data@%i.volume`. Quadlet will read the template file
+and determine the correct volume name (with the systemd specifier) for
+the `podman run` command in the generated `.service` file. In this example,
+the `ExecStart` generated by Quadlet would look something like this:
+
+```
+ExecStart=/usr/bin/podman run ... -v my-name-%i:/mnt/path ...
+```
+
 ### Relative paths
 
 In order to support Systemd specifiers, Quadlet does not resolve relative paths that start with `%`.