docs/plugin-protocol: Add notes about missing configuration in ReadResource and UpgradeResourceState request messages (#31998)

This opts to inline document these intentional design decisions in the protocol definition as a catch-all for it not being documented elsewhere.

Protocol Buffers files updated via:

```shell
make protobuf
```

docs/plugin-protocol/tfplugin5.3.proto

@@ -183,6 +183,15 @@ message PrepareProviderConfig {
 }
 
 message UpgradeResourceState {
+    // Request is the message that is sent to the provider during the
+    // UpgradeResourceState RPC.
+    //
+    // This message intentionally does not include configuration data as any
+    // configuration-based or configuration-conditional changes should occur
+    // during the PlanResourceChange RPC. Additionally, the configuration is
+    // not guaranteed to exist (in the case of resource destruction), be wholly
+    // known, nor match the given prior state, which could lead to unexpected
+    // provider behaviors for practitioners.
     message Request {
         string type_name = 1;
 
@@ -240,6 +249,14 @@ message Configure {
 }
 
 message ReadResource {
+    // Request is the message that is sent to the provider during the
+    // ReadResource RPC.
+    //
+    // This message intentionally does not include configuration data as any
+    // configuration-based or configuration-conditional changes should occur
+    // during the PlanResourceChange RPC. Additionally, the configuration is
+    // not guaranteed to be wholly known nor match the given prior state, which
+    // could lead to unexpected provider behaviors for practitioners.
     message Request {
         string type_name = 1;
         DynamicValue current_state = 2;

docs/plugin-protocol/tfplugin6.3.proto

@@ -201,6 +201,15 @@ message ValidateProviderConfig {
 }
 
 message UpgradeResourceState {
+    // Request is the message that is sent to the provider during the
+    // UpgradeResourceState RPC.
+    //
+    // This message intentionally does not include configuration data as any
+    // configuration-based or configuration-conditional changes should occur
+    // during the PlanResourceChange RPC. Additionally, the configuration is
+    // not guaranteed to exist (in the case of resource destruction), be wholly
+    // known, nor match the given prior state, which could lead to unexpected
+    // provider behaviors for practitioners.
     message Request {
         string type_name = 1;
 
@@ -258,6 +267,14 @@ message ConfigureProvider {
 }
 
 message ReadResource {
+    // Request is the message that is sent to the provider during the
+    // ReadResource RPC.
+    //
+    // This message intentionally does not include configuration data as any
+    // configuration-based or configuration-conditional changes should occur
+    // during the PlanResourceChange RPC. Additionally, the configuration is
+    // not guaranteed to be wholly known nor match the given prior state, which
+    // could lead to unexpected provider behaviors for practitioners.
     message Request {
         string type_name = 1;
         DynamicValue current_state = 2;

internal/tfplugin5/tfplugin5.pb.go

@@ -1800,6 +1800,15 @@ func (x *PrepareProviderConfig_Response) GetDiagnostics() []*Diagnostic {
 	return nil
 }
 
+// Request is the message that is sent to the provider during the
+// UpgradeResourceState RPC.
+//
+// This message intentionally does not include configuration data as any
+// configuration-based or configuration-conditional changes should occur
+// during the PlanResourceChange RPC. Additionally, the configuration is
+// not guaranteed to exist (in the case of resource destruction), be wholly
+// known, nor match the given prior state, which could lead to unexpected
+// provider behaviors for practitioners.
 type UpgradeResourceState_Request struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -2236,6 +2245,14 @@ func (x *Configure_Response) GetDiagnostics() []*Diagnostic {
 	return nil
 }
 
+// Request is the message that is sent to the provider during the
+// ReadResource RPC.
+//
+// This message intentionally does not include configuration data as any
+// configuration-based or configuration-conditional changes should occur
+// during the PlanResourceChange RPC. Additionally, the configuration is
+// not guaranteed to be wholly known nor match the given prior state, which
+// could lead to unexpected provider behaviors for practitioners.
 type ReadResource_Request struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache

internal/tfplugin6/tfplugin6.pb.go

@@ -1819,6 +1819,15 @@ func (x *ValidateProviderConfig_Response) GetDiagnostics() []*Diagnostic {
 	return nil
 }
 
+// Request is the message that is sent to the provider during the
+// UpgradeResourceState RPC.
+//
+// This message intentionally does not include configuration data as any
+// configuration-based or configuration-conditional changes should occur
+// during the PlanResourceChange RPC. Additionally, the configuration is
+// not guaranteed to exist (in the case of resource destruction), be wholly
+// known, nor match the given prior state, which could lead to unexpected
+// provider behaviors for practitioners.
 type UpgradeResourceState_Request struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -2255,6 +2264,14 @@ func (x *ConfigureProvider_Response) GetDiagnostics() []*Diagnostic {
 	return nil
 }
 
+// Request is the message that is sent to the provider during the
+// ReadResource RPC.
+//
+// This message intentionally does not include configuration data as any
+// configuration-based or configuration-conditional changes should occur
+// during the PlanResourceChange RPC. Additionally, the configuration is
+// not guaranteed to be wholly known nor match the given prior state, which
+// could lead to unexpected provider behaviors for practitioners.
 type ReadResource_Request struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache