`<relabel_config>` -> <relabel_action> description not comlete for `drop` and `keep`

[thernstig]

From https://prometheus.io/docs/prometheus/latest/configuration/configuration/#relabel_config:

keep: Drop targets for which regex does not match the concatenated source_labels.
drop: Drop targets for which regex matches the concatenated source_labels.

This incorrectly says that drop/keep is only to drop targets, when it is also to drop/keep metrics when used with <metric_relabel_configs>

This is slightly related to #341, but #341 says that relabel_config is accurate which is not true.