Minor documentation update - explicit mention of PID in GenServer.call/3

[silverdr]

This PR comes as a result of discussion in elixirforum thread where the documentation for GenServer.call/3 was found to be lacking explicit mention of PID as a valid value for the server argument. Documentation sends reader to "Name registration" section, which in its vast majority covers the subject of naming and registering GenServers. While it in fact mentions the possibility of using PID in one of the paragraphs, it is arguably better to make a distinction as PID is neither a "name" nor something that the user can "register". This change should make it clear for situations where only PID is available, without deep referring to another section of the documentation, potentially overlooking the single mention of PID there.

[josevalim]

Thank you! If there are other places which could benefit from the same change, another PR is welcome!