add variable gotcha to the variables docs not just the from-json hcl guides.

website/pages/docs/from-1.5/variables.mdx

@@ -168,6 +168,72 @@ $ packer build -var='image_id_map={"us-east-1":"ami-abc123","us-east-2":"ami-def
 
 The `-var` option can be used any number of times in a single command.
 
+If you plan to assign variables via the command line, we strongly recommend that
+you at least set a default type instead of using empty blocks; this helps the
+HCL parser understand what is being set.
+
+For example:
+
+```hcl
+variable "pizza" {
+  type = string
+}
+source "null" "example" {
+    communicator = "none"
+}
+build {
+  sources = [
+    "source.null.example"
+  ]
+  provisioner "shell-local" {
+    inline = ["echo $PIZZA"]
+    environment_vars = ["PIZZA=${var.pizza}"]
+  }
+}
+```
+
+If you call the above template using the command
+
+```sh
+packer build -var pizza=pineapple shell_local_variables.pkr.hcl
+```
+
+then the Packer build will run successfully. However, if you define the variable
+using an empty block, the parser will not know what type the variable is, and it
+cannot infer the type from the command line, as shown in this example:
+
+```hcl
+variable "pizza" {}
+source "null" "example" {
+    communicator = "none"
+}
+build {
+  sources = [
+    "source.null.example"
+  ]
+  provisioner "shell-local" {
+    inline = ["echo $PIZZA"]
+    environment_vars = ["PIZZA=${var.pizza}"]
+  }
+}
+```
+The above template will result in the error:
+
+```
+Error: Variables not allowed
+   on <value for var.pizza from arguments> line 1:
+   (source code not available)
+Variables may not be used here.
+```
+
+You can work around this either by quoting the variable on the command line, or
+by adding the type to the variable block as shown in the previous example.
+Setting the expected type is the more resilient option.
+
+```sh
+packer build -var 'pizza="pineapple"' shell_local_variables.pkr.hcl
+```
+
 ### Variable Definitions (`.pkrvars.hcl`) Files
 
 To set lots of variables, it is more convenient to specify their values in a