lib: Update hash.h documentation

lib: Update hash.h documentation

The hash_get function when called and the alloc_func returns
a NULL value, we do not create a backet nor do we insert
anything into the hash.  As such backet->data must always
be non-NULL.

Modify the description in hash_get to inform of this.
Additionally indicate that hash_walk and hash_iterate
cannot have a NULL backet->data value.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>