Move Zihintpause rationale to appendix (#2371)

Fixes #2370

Author: aswaterman

src/rationale.adoc

@@ -12,6 +12,23 @@ the rationale will be added over time.
 In cases where the rationale was not recorded, the authors and editors will
 synthesize it from the historical record.
 
+=== "Zihintpause" Extension for Pause Hint
+
+The PAUSE instruction hints to a hart that it should temporarily reduce its
+rate of execution.
+It is normally used to save energy and execution resources while polling, e.g.
+while waiting for a spinlock to become free.
+
+Much of the debate surrounding this extension centered on whether a facility
+similar to x86's MONITOR/MWAIT should instead be provided.
+We concluded that, even if such a facility were to be defined for RISC-V,
+it would not supplant PAUSE.
+PAUSE is more appropriate when polling for non-memory events, when polling for
+multiple events, or when software does not know precisely what events it is
+polling for.
+(Perhaps surprisingly, the latter case is ubiquitous, in part because it is
+the mechanism expected by the Linux kernel's `cpu_relax` API.)
+
 === "Zicond" Extension for Integer Conditional Operations
 
 Replacing unpredictable branches with conditional-select or conditional-move

src/zihintpause.adoc

@@ -15,13 +15,6 @@ executed. It is recommended that a PAUSE instruction generally be
 included in the code sequence for a spin-wait loop.
 (((PAUSE, energy consumption)))
 
-A future extension might add primitives similar to the x86 MONITOR/MWAIT
-instructions, which provide a more efficient mechanism to wait on writes
-to a specific memory location. However, these instructions would not
-supplant PAUSE. PAUSE is more appropriate when polling for non-memory
-events, when polling for multiple events, or when software does not know
-precisely what events it is polling for.
-
 The duration of a PAUSE instruction's effect may vary significantly
 within and among implementations. In typical implementations this
 duration should be much less than the time to perform a context switch,