of: base: improve of_get_next_child() kernel-doc

Add two new facts to of_get_next_child() documentation:

   * of_get_next_child() returns NULL when there is not next child

   * of_get_next_child() decrements the refcount of prev

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Rob Herring <robh@kernel.org>