doc: environment: Expand on fdt_addr, initrd_addr and loadaddr

- Explain why fdt_addr and initrd_addr should not be set to disable
  relocation normally.
- Provide some advice on the typical loadaddr default value.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed