Document standard Log::Any::Adapter API

[preaction]

If a user wants to use an adapter directly, for example, to move log messages from one logging system to the Log::Any::Adapter, we should support this. To this end, we should document the Adapter API in, for example, Log::Any::Adapter::Base (with a link from Log::Any::Adapter explaining).

This document should explain all the methods that adapters are required to have by default, the arguments they are required to have, and any return values they are required to have.