When you start to argue something on the basis of a signature by a key, they only next step can be some knowledge about the key. In the semantic web, this is a processing rule about things which are signed with that key. Documents have meaning. Signatures by themselves do not. So it is not useful to ask what the semantics of a signature are.
Signatures convey trust, but even that because of a set of statements about keys and documents.
There are in society many rules about the trust which is conveyed by the signature under various circumstances. We should not attempt to model those when we make the basic infrastructure of the semantic web.