nixos/tmp: add a note to useTmpfs on potential issues

nixos/modules/system/boot/tmp.nix

···
         default = false;
         description = lib.mdDoc ''
            Whether to mount a tmpfs on {file}`/tmp` during boot.
+
+           ::: {.note}
+           Large Nix builds can fail if the mounted tmpfs is not large enough.
+           In such a case either increase the tmpfsSize or disable this option.
+           :::
         '';
       };
     };