Document log level format (#1982)

Specify log levels that can be configured with Athens explicitly in the documentation.

docs/content/configuration/logging.md

@@ -10,6 +10,8 @@ Athens is designed to support a myriad of logging scenarios.
 
 The standard structured logger can be configured in `plain` or `json` formatting via `LogFormat` or `ATHENS_LOG_FORMAT`. Additionally, verbosity can be controlled by setting `LogLevel` or `ATHENS_LOG_LEVEL`. In order for the standard structured logger to work, `CloudRuntime` and `ATHENS_CLOUD_RUNTIME` should not be set to a valid value.
 
+The logging is via [Logrus](https://github.com/sirupsen/logrus), so the allowed values for logging config options are determined by that project. For example, `ATHENS_LOG_LEVEL` can be `debug`, `info`, `warn`/`warning`, `error`, etc.
+
 ## Runtimes
 
-Athens can be configured according to certain cloud provider specific runtimes. The **GCP** runtime configures Athens to rename certain logging fields that could be dropped or overriden when running in a GCP logging environment. This runtime can be used with `LogLevel` or `ATHENS_LOG_LEVEL` to control the verbosity of logs.
+Athens can be configured according to certain cloud provider specific runtimes. The **GCP** runtime configures Athens to rename certain logging fields that could be dropped or overriden when running in a GCP logging environment. This runtime can be used with `LogLevel` or `ATHENS_LOG_LEVEL` to control the verbosity of logs.