docs(comment): Explain why we use reformatter for terraform-mode

Terraform-mode already comes with it's own
terraform-format-on-save-mode, but the way it inserts the corrected
source into the buffer often causes weird cursor movement which is
annoying.

modules/languages/siren-terraform.el

@@ -21,6 +21,8 @@
     (terraform-format-on-save-mode 1))
 
   :config
+  ;; This does a better job of injecting formatted content than the default
+  ;; formatting commands included with terraform-mode.
   (reformatter-define terraform-format
     :program "terraform"
     :args '("fmt" "-no-color" "-")