Add details on the expected contents of a variable definitions file (#11403)

Closes #10903

website/content/guides/hcl/variables.mdx

@@ -171,15 +171,19 @@ command line variables as strings.
 #### From a file
 
 To persist variable values, create a file and assign variables within
-this file. Create a file named `variables.pkrvars.hcl` with the following
-contents:
+this file. A variable definitions file uses the same basic syntax as Packer language files,
+but consists only of variable name assignments.
 
 ```hcl
 sudo_password = "partyparrot"
 weekday       = "Sunday"
 ```
 
-You tell Packer to use this var file using the `-var-file` command line flag.
+When assigning values in a variable definitions file please make sure to quote any string values,
+wrap list values within brackets `[...]`, and wrap maps within curly-braces `{...}`.
+
+Given a file named `variables.pkrvars.hcl` Packer can be instructed to use the
+variable definitions file by using the `-var-file` command line flag.
 
 ```shell-session
 $ packer build -var-file="variables.pkrvars.hcl" .
@@ -187,8 +191,8 @@ $ packer build -var-file="variables.pkrvars.hcl" .
 
 Packer will automatically load any var file that matches the name
 `*.auto.pkrvars.hcl`, without the need to pass the file via the command line.
-if we rename the above var-file from variables.pkrvars.hcl to
-variables.auto.pkrvars.hcl, then we can run our build simply by calling
+If we rename the above variable definitions file from `variables.pkrvars.hcl` to
+`variables.auto.pkrvars.hcl`, then we can run our build simply by calling
 
 ```shell-session
 $ packer build .