This doc has a few pieces that are out-of-date. Fix these. Also we have
started to use 'devicetree' instead of 'device tree' or 'device-tree'
since it is easier to see as a single term, so replace all ocurrences
accordingly.

Also move the caveats to the end, since this is a fairly solid part of
U-Boot now.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>