esp32: add some implementation comments

components/esp32/hw_random.c

@@ -23,6 +23,14 @@
 
 uint32_t IRAM_ATTR esp_random(void)
 {
+    /* The PRNG which implements WDEV_RANDOM register gets 2 bits
+     * of extra entropy from a hardware randomness source every APB clock cycle.
+     * To make sure entropy is not drained faster than it is added,
+     * this function needs to wait for at least 16 APB clock cycles after reading
+     * previous word. This implementation may actually wait a bit longer
+     * due to extra time spent in arithmetic and branch statements.
+     */
+
     static uint32_t last_ccount = 0;
     uint32_t ccount;
     do {