list/hashtable: minor documentation corrections.

hash_for_each_safe() and hash_for_each_possible_safe()
need to be passed a temp 'struct hlist_node' pointer, but
do not say that in the documentation - they just say
a 'struct'.

Also the documentation for hlist_for_each_entry_safe()
describes @n as "another" hlist_node, but in reality it is
the only one.

Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>