update custon provider docs

website/source/docs/extending/custom-builders.html.md

@@ -31,7 +31,8 @@ method should do.
 
 ``` go
 type Builder interface {
-  Prepare(...interface{}) error
+  ConfigSpec() hcldec.ObjectSpec
+  Prepare(...interface{}) ([]string, []string, error)
   Run(context.Context, ui Ui, hook Hook) (Artifact, error)
 }
 ```

website/source/docs/extending/custom-post-processors.html.md

@@ -36,6 +36,7 @@ explaining what each method should do.
 
 ``` go
 type PostProcessor interface {
+  ConfigSpec() hcldec.ObjectSpec
   Configure(interface{}) error
   PostProcess(context.Context, Ui, Artifact) (a Artifact, keep, mustKeep bool, err error)
 }

website/source/docs/extending/custom-provisioners.html.md

@@ -35,8 +35,9 @@ explaining what each method should do.
 
 ``` go
 type Provisioner interface {
+  ConfigSpec() hcldec.ObjectSpec
   Prepare(...interface{}) error
-  Provision(Ctx, Ui, Communicator, new(interface{})) error
+  Provision(context.Context, Ui, Communicator, map[string]interface{}) error
 }
 ```