docs: Add clarification about all lowercase custom data fields

Hi, 

I'm new to chezmoi and started to look into templates. I've spent quite some time to figure out why I could not get custom variables in the `data` section of the config to work until I realized that I've stumbled upon twpayne#463.

I propose to add a hint or disclaimer to the documentation here to make it easier for newcomers to spot this pitfall (and avoid it).

Please let me know if I need to change or update anything.

assets/chezmoi.io/docs/reference/templates/variables.md

@@ -28,3 +28,8 @@ chezmoi provides the following automatically-populated variables:
 Additional variables can be defined in the config file in the `data` section.
 Variable names must consist of a letter and be followed by zero or more letters
 and/or digits.
+
+!!! hint
+
+    Until [#463](https://github.com/twpayne/chezmoi/issues/463) is resolved, custom
+    data fields from the `data` section appear as all lowercase strings.