document log level and format

Signed-off-by: Oleg Atamanenko <oleg.atamanenko@gmail.com>

docs/man/containerd-config.toml.5.md

@@ -63,7 +63,9 @@ config as version 1 has been deprecated.
 - **address** (Default: "/run/containerd/debug.sock")
 - **uid** (Default: 0)
 - **gid** (Default: 0)
-- **level** (Default: "info") sets the debug log level
+- **level** (Default: "info") sets the debug log level. Supported levels are:
+  "trace", "debug", "info", "warn", "error", "fatal", "panic"
+- **format** (Default: "text") sets log format. Supported formats are "text" and "json"
 
 **[metrics]**
 : Section to enable and configure a metrics listener. Contains two properties:

services/server/config/config.go

@@ -146,7 +146,7 @@ type Debug struct {
 	UID     int    `toml:"uid"`
 	GID     int    `toml:"gid"`
 	Level   string `toml:"level"`
-	// Format represents the logging format
+	// Format represents the logging format. Supported values are 'text' and 'json'.
 	Format string `toml:"format"`
 }