Attempts to access non-existent CSRs are reserved

Resolves #1116

src/priv-csrs.adoc

@@ -42,9 +42,9 @@ accesses to be intercepted. This change should be transparent to the
 less-privileged software.
 ====
 
-Attempts to access a non-existent CSR raise an illegal-instruction
-exception. Attempts to access a CSR without appropriate privilege level
-or to write a read-only register also raise illegal-instruction
+Instructions that access a non-existent CSR are reserved.
+Attempts to access a CSR without appropriate privilege level
+or to write a read-only register raise illegal-instruction
 exceptions. A read/write register might also contain some bits that are
 read-only, in which case writes to the read-only bits are ignored.