Adds additional godoc comments and CHANGELOG

CHANGELOG.md

@@ -1,5 +1,7 @@
 # UNRELEASED
 
+* Adds more BETA support for `Stacks` resources, which is EXPERIMENTAL, SUBJECT TO CHANGE, and may not be available to all users by @brandonc. [#934](https://github.com/hashicorp/go-tfe/pull/934)
+
 # v1.59.0
 
 ## Features

stack_configuration.go

@@ -6,6 +6,10 @@ import (
 	"net/url"
 )
 
+// StackConfigurations describes all the stacks configurations-related methods that the
+// HCP Terraform API supports.
+// NOTE WELL: This is a beta feature and is subject to change until noted otherwise in the
+// release notes.
 type StackConfigurations interface {
 	// ReadConfiguration returns a stack configuration by its ID.
 	Read(ctx context.Context, ID string) (*StackConfiguration, error)

stack_deployments.go

@@ -6,6 +6,10 @@ import (
 	"net/url"
 )
 
+// StackDeployments describes all the stacks deployments-related methods that the
+// HCP Terraform API supports.
+// NOTE WELL: This is a beta feature and is subject to change until noted otherwise in the
+// release notes.
 type StackDeployments interface {
 	// Read returns a stack deployment by its name.
 	Read(ctx context.Context, stackID, deployment string) (*StackDeployment, error)