core: Document postconditions as valid use of self

This is not currently gated by the experiment only because it is awkward
to do so in the context of evaluationStateData, which doesn't have any
concept of experiments at the moment.

internal/lang/eval.go

@@ -296,7 +296,7 @@ func (s *Scope) evalContext(refs []*addrs.Reference, selfAddr addrs.Referenceabl
 					// this codepath doesn't really "know about". If the "self"
 					// object starts being supported in more contexts later then
 					// we'll need to adjust this message.
-					Detail:  `The "self" object is not available in this context. This object can be used only in resource provisioner and connection blocks.`,
+					Detail:  `The "self" object is not available in this context. This object can be used only in resource provisioner, connection, and postcondition blocks.`,
 					Subject: ref.SourceRange.ToHCL().Ptr(),
 				})
 				continue

internal/terraform/evaluate_valid.go

@@ -58,7 +58,7 @@ func (d *evaluationStateData) staticValidateReference(ref *addrs.Reference, self
 				// this codepath doesn't really "know about". If the "self"
 				// object starts being supported in more contexts later then
 				// we'll need to adjust this message.
-				Detail:  `The "self" object is not available in this context. This object can be used only in resource provisioner and connection blocks.`,
+				Detail:  `The "self" object is not available in this context. This object can be used only in resource provisioner, connection, and postcondition blocks.`,
 				Subject: ref.SourceRange.ToHCL().Ptr(),
 			})
 			return diags