libfdt: Add more documentation (patch the third)

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> libfdt: Add more documentation (patch the third)
> 
> This patch adds documentation in libfdt.h for a few more libfdt
> functions.  It also makes a slight update to the documentation of
> fdt_get_name().
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl