xfrm: minor update to sdb and xfrm_policy comments

The spd is no longer maintained as a linear list.
We also haven't been caching bundles in the xfrm_policy
struct since 2010.

While at it, add kdoc style comments for the xfrm_policy structure
and extend the description of the current rbtree based search to
mention why it needs to search the candidate set.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>