Add back comments about Stable/familiar decay

Closes: #40734.

src/feature/dirauth/dirauth_options.inc

@@ -86,11 +86,12 @@ CONF_VAR(AuthDirVoteGuard, ROUTERSET, 0, NULL)
 CONF_VAR(AuthDirVoteStableGuaranteeMinUptime, INTERVAL, 0, "30 days")
 
 /** If a relay's MTBF is at least this value, then it is always stable. See
- * above. */
+ * above. (Corresponds to about 7 days for current decay rates.) */
 CONF_VAR(AuthDirVoteStableGuaranteeMTBF, INTERVAL, 0, "5 days")
 
 /** A relay with at least this much weighted time known can be considered
- * familiar enough to be a guard. */
+ * familiar enough to be a guard. (Corresponds to about 20 days for current
+ * decay rates.) */
 CONF_VAR(AuthDirVoteGuardGuaranteeTimeKnown, INTERVAL, 0, "8 days")
 
 /** A relay with sufficient WFU is around enough to be a guard. */