manual: Clarify that limits.conf doesn't apply to systemd services. (#40267)

Say how systemd services can be set instead.

Sources:

* https://bugzilla.redhat.com/show_bug.cgi?id=754285
* https://ro-che.info/articles/2017-03-26-increase-open-files-limit

Signed-off-by: Niklas Hambüchen <[e-mail]>

nixos/modules/security/pam.nix

···
           <varname>item</varname>, and <varname>value</varname>
           attribute.  The syntax and semantics of these attributes
           must be that described in the limits.conf(5) man page.
+
+          Note that these limits do not apply to systemd services,
+          whose limits can be changed via <option>systemd.extraConfig</option>
+          instead.
        '';
     };
 

nixos/modules/system/boot/coredump.nix

···
           Enables storing core dumps in systemd.
           Note that this alone is not enough to enable core dumps. The maximum
           file size for core dumps must be specified in limits.conf as well. See
-          <option>security.pam.loginLimits</option> as well as the limits.conf(5)
-          man page.
+          <option>security.pam.loginLimits</option> and the limits.conf(5)
+          man page (these specify the core dump limits for user login sessions)
+          and <option>systemd.extraConfig</option> (where e.g.
+          <literal>DefaultLimitCORE=1000000</literal> can be specified to set
+          the core dump limit for systemd system-level services).
         '';
       };