Add clarifying non-normative comments about [m]time (riscv#1833)

Text contributed by @gfavor

Author: aswaterman

src/counters.adoc

@@ -147,6 +147,22 @@ As with other architectural mandates, it suffices to appear "as if"
 harts are synchronized to within one tick of the real-time clock, i.e.,
 software is unable to observe that there is a greater delta between the
 real-time clock values observed on two harts.
+
+If, for example, the real-time clock increments at a frequency of 1 GHz, then
+all harts must appear to be synchronized to within 1 nsec.
+But it is also acceptable for this example implementation to only update the
+real-time clock at, say, a frequency of 100 MHz with increments of 10 ticks.
+As long as software cannot observe this seeming violation of the above
+synchronization requirement, and software always observes time across harts to
+be monotonically nondecreasing, then this implementation is compliant.
+
+A platform spec may then, for example, specify an apparent real-time clock
+tick frequency (e.g. 1 GHz) and also a minimum update frequency (e.g. 100 MHz)
+at which updated time values are guaranteed to be observable by software.
+Software may read time more frequently, but it should only observe
+monotonically nondecreasing values and it should observe a new value at least
+once every 10 ns (corresponding to the 100 MHz update frequency in this
+example).
 ====
 The RDINSTRET pseudoinstruction reads the low XLEN bits of the
 `instret` CSR, which counts the number of instructions retired by this