Clarify that protection checks apply to failed SC (riscv#1893)

Resolves riscv#1892

Author: aswaterman

src/a-st-ext.adoc

@@ -66,8 +66,14 @@ if the reservation is still valid and the reservation set contains the
 bytes being written. If the SC.W succeeds, the instruction writes the
 word in _rs2_ to memory, and it writes zero to _rd_. If the SC.W fails,
 the instruction does not write to memory, and it writes a nonzero value
-to _rd_.  For the purposes of memory protection, a failed SC.W may be
-treated like a store.  Regardless of success or failure, executing an
+to _rd_.
+No SC.W instruction shall retire unless it passes memory permission checks,
+but it is UNSPECIFIED whether any side effects of implicit address translation
+and protection memory accesses (such as setting a page-table entry D bit)
+occur on a failed SC.W.
+For the purposes of memory protection, a failed SC.W may be
+treated like a store.
+Regardless of success or failure, executing an
 SC.W instruction invalidates any reservation held by this hart. LR.D and
 SC.D act analogously on doublewords and are only available on RV64. For
 RV64, LR.W and SC.W sign-extend the value placed in _rd_.