SGX heartbeat documentation (#259)

- Added note to heartbeat documentation stating that it is currently unsupported for SGX
- Added corresponding unsupported notes in protocol operations within the protocol documentation

docs/heartbeat.md

@@ -2,7 +2,7 @@
 
 ## Foreword
 
-Currently, just like what happens in the case of [attestation](./attestation.md), heartbeat is a feature supported only in the Ledger version of powHSM. A heartbeat implementation for the SGX version of powHSM is currently under development. Therefore, all the information contained herein must be interpreted as applying exclusively to the Ledger version of powHSM.
+Currently, heartbeat is a feature supported only in the Ledger version of powHSM. A heartbeat implementation for the SGX version of powHSM is currently under development. Therefore, all the information contained herein must be interpreted as applying exclusively to the Ledger version of powHSM.
 
 ## Abstract
 

docs/protocol.md

@@ -356,6 +356,8 @@ This operation can return `0` and generic errors. See the error codes section fo
 
 **Error codes:**
 This operation can return `0`, `-301` and generic errors. See the error codes section for details.
+**Important:**
+Currently, this operation is unsupported for the SGX version of powHSM, returning error `-905` upon an otherwise correct invocation.
 
 ### UI heartbeat
 
@@ -390,6 +392,8 @@ This operation can return `0`, `-301` and generic errors. See the error codes se
 
 **Error codes:**
 This operation can return `0`, `-301` and generic errors. See the error codes section for details.
+**Important:**
+Currently, this operation is unsupported for the SGX version of powHSM, returning error `-905` upon an otherwise correct invocation.
 
 ### Error and success codes