Fix phpdoc and function scope

Well @stof is wright, `element` is really not clear. But, ideally a `KnowsMetadata` defined a part of an  `Adapter` but it's not a `composition` of an adapter. We can use `KnowsMetadata` to qualify something else than an adapter.

Anyway, we need to find an other word than `element`. Maybe, like i said, `part of adapter` ?