Clarify that mtimecmp comparison is unsigned

src/machine.tex

@@ -1610,9 +1610,10 @@ \subsection{Machine Timer Registers ({\tt mtime} and {\tt mtimecmp})}
 
 The {\tt mtime} register has a 64-bit precision on all RV32 and RV64
 systems.  Platforms provide a 64-bit memory-mapped machine-mode
-timer compare register ({\tt mtimecmp}), which causes a timer
-interrupt to be posted when the {\tt mtime} register contains a value
-greater than or equal to the value in the {\tt mtimecmp} register.
+timer compare register ({\tt mtimecmp}).
+A timer interrupt becomes pending whenever {\tt mtime} contains
+a value greater than or equal to {\tt mtimecmp}, treating the values
+as unsigned integers.
 The interrupt remains posted until {\tt mtimecmp} becomes greater than
 {\tt mtime} (typically as a result of writing {\tt mtimecmp}).
 The interrupt will only be taken if interrupts