Add deprecation note for default prefix and suffix in provider documentation

[gessnerfl]

For completeness, update the documentation to mark default prefix and suffix as deprecated in the documentation. In the provider schema the configuration was already adopted.

[gessnerfl]

completed with commit https://github.com/gessnerfl/terraform-provider-instana/commit/5855da62d8bf22c86c007294eee6ef2f040e6836