docs: document helm fileParameters / helm-set-file

Added documentation for fileParameters in a helm source Application or
the --helm-set-file cli option from argoproj#2751. Added a note about argoproj#13220 so
users are aware. argoproj#17941 aims to remove this limitation so the warning
will be adjusted to mention the version that lands that support.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>

docs/user-guide/helm.md

@@ -201,6 +201,28 @@ the result will be param1=value5
     The list of parameters seen in the ui is not what is used for resources, rather it is the values/valuesObject merged with parameters (see [this issue](https://github.com/argoproj/argo-cd/issues/9213) incase it has been resolved)
     As a workaround using parameters instead of values/valuesObject will provide a better overview of what will be used for resources
 
+## Helm --set-file support
+
+The `--set-file` argument to helm can be used with the following syntax on
+the cli:
+
+```bash
+argocd app set helm-guestbook --helm-set-file some.key=path/to/file.ext
+```
+
+or using the fileParameters for yaml:
+
+```yaml
+source:
+  helm:
+    fileParameters:
+      - name: some.key
+        value: path/to/file.ext
+```
+
+!!! warning "Reference in multiple sources not supported"
+    Please note that using a multiple sources application will not let you load the file by reference. See [argoproj/argo-cd#13220](https://github.com/argoproj/argo-cd/issues/13220)
+
 ## Helm Release Name
 
 By default, the Helm release name is equal to the Application name to which it belongs. Sometimes, especially on a centralised Argo CD,