add comments from @Varriount regarding ARM, refs nim-lang/RFCs#411 (c…

…omment)

lib/std/cputicks.nim

@@ -7,6 +7,23 @@ Future work:
 * convert ticks to time; see some approaches here: https://quick-bench.com/q/WcbqUWBCoNBJvCP4n8h3kYfZDXU
 * provide feature detection to test whether the CPU supports it (on linux, via /proc/cpuinfo)
 
+## js
+* we use `window.performance.now()`
+
+## nodejs
+* we use `process.hrtime.bigint()`
+
+## ARM
+* The ARMv8-A architecture[1] manual explicitly states that two reads to the PMCCNTR_EL0 register may return the same value[1a].
+  There is also the  CNTVCT_EL0[1b] register, however it's unclear whether that register is even monotonic (it's implied, but not stated explicitly).
+  The ARMv8-M architecture[2] has the CYCCNT register, however all that's mentioned is that it is an "optional free-running 32-bit cycle counter"[2a].
+
+## references
+[1] https://documentation-service.arm.com/static/611fa684674a052ae36c7c91
+[1a] See [1], PDF page 2852
+[2] https://documentation-service.arm.com/static/60e6f8573d73a34b640e0cee
+[2a] See [2]. PDF page 367
+
 ## further links
 * https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/ia-32-ia-64-benchmark-code-execution-paper.pdf
 * https://gist.github.com/savanovich/f07eda9dba9300eb9ccf