clarify CSRs which are only added if hybrid mode is implemented (#503)

src/attributes.adoc

@@ -69,6 +69,7 @@ endif::[]
 
 :TAG_RESET_CSR: The tag of the CSR must be reset to zero. The reset values of the metadata and address fields are UNSPECIFIED.
 :REQUIRE_CRE_CSR: Access to this CSR is illegal if <<section_cheri_disable,CHERI register and instruction access is disabled>> for the current privilege.
+:REQUIRE_HYBRID_CSR: This CSR is only implemented if {cheri_default_ext_name} is implemented.
 
 :CAP_MODE_VALUE: 0
 :INT_MODE_VALUE: 1

src/hypervisor-integration.adoc

@@ -157,6 +157,8 @@ present when the implementation supports {cheri_default_ext_name}.
 
 {REQUIRE_CRE_CSR}
 
+{REQUIRE_HYBRID_CSR}
+
 .Virtual supervisor trap default capability register
 include::img/vstdcreg.edn[]
 

src/riscv-hybrid-integration.adoc

@@ -298,6 +298,8 @@ xref:default-csrnames-added[xrefstyle=short].
 
 {TAG_RESET_CSR}
 
+{REQUIRE_HYBRID_CSR}
+
 .Debug default data capability
 include::img/dddcreg.edn[]
 
@@ -430,6 +432,8 @@ machine-mode hart-local context space, to load into <<ddc>>.
 
 {REQUIRE_CRE_CSR}
 
+{REQUIRE_HYBRID_CSR}
+
 .Machine-mode trap data capability register
 include::img/mtdcreg.edn[]
 
@@ -481,6 +485,8 @@ a supervisor-mode hart-local context space, to load into <<ddc>>.
 
 {REQUIRE_CRE_CSR}
 
+{REQUIRE_HYBRID_CSR}
+
 .Supervisor trap data capability register (*stdc*)
 include::img/stdcreg.edn[]
 
@@ -516,6 +522,8 @@ is the <<infinite-cap>> capability.
 
 NOTE: CRE is not required for the implicit access required by checking memory accesses against <<ddc>>
 
+{REQUIRE_HYBRID_CSR}
+
 As shown in xref:CSR_exevectors[xrefstyle=short], <<ddc>> is a data pointer,
 so it does not need to be able to hold all possible invalid addresses.