Fix the LokiPushApi docs

[dstathis]

Enhancement Proposal

The docs for LokiPushApiConsumer say for the relation_name argument:

This argument is not required if your metadata.yaml has precisely one
required relation in metadata.yaml with the `loki_push_api` interface, as the
lib will automatically resolve the relation name inspecting the using the
meta information of the charm

but in fact it just uses logging

[dstathis]

Also, we should skim through and make sure everything else looks correct.