Update HCP Terraform Secrets Engine docs to clarify API token types, scope limitations, and usage examples

[kevin-loehfelm]

Current Terraform Secrets Engine documentation here is misleading:

• The documentation recommends using a user API token as the root/anchor credential, but does not call out the limitation that user tokens cannot create user tokens for other users. This could lead to a misunderstanding that the Terraform Secrets engine can create short lived user API tokens
• The example shows a user API token for the root/anchor credential and creation of a user role, but does not call out that this would only work for the root/anchor user