add terraform fmt fixer for terraform and hcl filetypes

See #1718

doc/ale-terraform.txt

@@ -2,6 +2,24 @@
 ALE Terraform Integration                               *ale-terraform-options*
 
 
+===============================================================================
+fmt                                                         *ale-terraform-fmt*
+
+g:ale_terraform_fmt_executable                 *g:ale_terraform_fmt_executable*
+                                               *b:ale_terraform_fmt_executable*
+
+  Type: |String|
+  Default: `'terraform'`
+
+  This variable can be changed to use a different executable for terraform.
+
+
+g:ale_terraform_fmt_options                       *g:ale_terraform_fmt_options*
+                                                  *b:ale_terraform_fmt_options*
+  Type: |String|
+  Default: `''`
+
+
 ===============================================================================
 tflint                                                   *ale-terraform-tflint*