tls_self_signed_cert reference subject doc from tls_cert_request

tls_self_signed_cert is really just a shorthand over tls_cert_request and
tls_locally_signed_cert, so rather than duplicating all of this
documentation and risking that it will get out of sync (since the
structure is shared in the implementation) we'll just link to the
existing docs.

This fixes hashicorp#5343.

Author: apparentlymart

website/source/docs/providers/tls/r/self_signed_cert.html.md

@@ -62,8 +62,9 @@ The following arguments are supported:
   an irreversable secure hash of the private key will be stored in the Terraform
   state.
 
-* `subject` - (Required) The subject for which a certificate is being requested. This is
-  a nested configuration block whose structure is described below.
+* `subject` - (Required) The subject for which a certificate is being requested.
+  This is a nested configuration block whose structure matches the
+  corresponding block for [`tls_cert_request`](cert_request.html).
 
 * `validity_period_hours` - (Required) The number of hours after initial issuing that the
   certificate will become invalid.