No, there is no document. Only some modules are located in initrd.
initrd image is under Linux_for_Tegra/bootloader
mkdir initrd; cd initrd;
sudo gunzip -c …/l4t_initrd.img.orig | sudo cpio -i
→ update the files you want to update
sudo find . | sudo cpio --quiet -H newc -o | sudo gzip -9 -n > …/l4t_initrd.img