libfdt: Add some documenting comments in libfdt.h

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch adds some internal documentation in libfdt.h, in the form
> of comments on the error codes and some functions.  Only a couple of
> functions are covered so far, leaving the documentation still woefully
> inadequate, but hey, it's a start.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

I did miss the fdt_next_tag patch earlier.
These are not the droids you are looking for.
Move along.

jdl