Tweak docs to properly document .tfvars format (hashicorp#6415)

As reported and described in hashicorp#3292

website/source/docs/configuration/variables.html.md

@@ -153,7 +153,13 @@ $ TF_VAR_image=foo terraform apply
 ## Variable Files
 
 Variables can be collected in files and passed all at once using the 
-`-var-file=foo` flag. 
+`-var-file=foo.tfvars` flag. The format for variables in `.tfvars`
+files is:
+```
+foo = "bar"
+xyz = "abc"
+
+```
 
 The flag can be used multiple times per command invocation:
 
@@ -165,22 +171,18 @@ terraform apply -var-file=foo.tfvars -var-file=bar.tfvars
 variable file (reading left to right) will be the definition used. Put more 
 simply, the last time a variable is defined is the one which will be used.
 
-##Example: 
+### Precedence example:
 
 Both these files have the variable `baz` defined:
 
 _foo.tfvars_
 ```
-variable "baz" { 
-    default = "foo"
-}
+baz = "foo"
 ```
 
 _bar.tfvars_
 ```
-variable "baz" { 
-    default = "bar"
-}
+baz = "bar"
 ```
 
 When they are passed in the following order: