docs: describe initrd= kernel option extraction

Change-Id: I2ada9b26609a52b8847ecb9cee0833198a9b6202
dtb is now built into apm mustang firmware, but still can get out of sync from kernel.

The default is to use the built in dtb if no dtb is specified in
grub.cfg. Use kernel option devicetree=path/under/tftpboot to specify
alternate dtb to use.

Bug: 1100008
Change-Id: Ia095337e27a7f2749fe97689b2dc046d99ee7aeb