journald service: Add helpful comments about the journal getting full

nixos/modules/system/boot/systemd.nix

@@ -593,6 +593,9 @@ in
         each other's limit. The value may be specified in the following
         units: s, min, h, ms, us. To turn off any kind of rate limiting,
         set either value to 0.
+
+        See <option>services.journald.rateLimitBurst</option> for important
+        considerations when setting this value.
       '';
     };
 
@@ -604,6 +607,19 @@ in
         interval) that is applied to all messages generated on the system.
         This rate limiting is applied per-service, so that two services
         which log do not interfere with each other's limit.
+
+        Note that the effective rate limit is multiplied by a factor derived
+        from the available free disk space for the journal as described on
+        <link xlink:href="https://www.freedesktop.org/software/systemd/man/journald.conf.html">
+        journald.conf(5)</link>.
+
+        Note that the total amount of logs stored is limited by journald settings
+        such as <literal>SystemMaxUse</literal>, which defaults to a 4 GB cap.
+
+        It is thus recommended to compute what period of time that you will be
+        able to store logs for when an application logs at full burst rate.
+        With default settings for log lines that are 100 Bytes long, this can
+        amount to just a few hours.
       '';
     };