core/weak_ptr: improve documentation
core/weak_ptr: improve documentation

Add documentation to the `weak_ptr` class (which previously had none)
and make the relation to `weakly_referencable` class clear, improving
the latter's discoverability. Previsouly this relation would only become
clear if one somehow ended up finding the `weakly_referencable` class
as well.
Also fix links to `weak_ptr` in the description of
`weakly_referencable` (drop the <> prefix).

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <54b2c4e1ebf52cd96a8cdedef0c7885099d23ab2.1543940445.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

dreamsxin/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar