website: Update api-docs for /ssh/sign/:name and /ssh/issue/:name (#17694)
website: Update api-docs for /ssh/sign/:name and /ssh/issue/:name (#17694)

Extend the documentation the API endpoint '/ssh/issue/:name' (added
in #15561 with v1.12.0) and '/ssh/issue/:name':

- Be more specific that the issued certificate uses the defaults
  given of the role at the given endpoint; and that it is subject
  to the limitations configured in this role.

- Note that the endpoint /ssh/issue/:name is available with v1.12+.

- Make it more clear that the generated credentials are only returned
  but not stored by Vault (not just the generated private key).
hashicorp/vault,hashicorp/vault,hashicorp/vault,hashicorp/vault,hashicorp/vault,hashicorp/vault