caddyfile: add documentation for new global log option

This corresponds with the changes in this PR:
caddyserver/caddy#4028

src/docs/markdown/caddyfile/options.md

@@ -32,6 +32,13 @@ Possible options are:
 		origins <origins...>
 		enforce_origin
 	}
+	log [name] {
+		output  <writer_module> ...
+		format  <encoder_module> ...
+		level   <level>
+		include <namespaces...>
+		exclude <namespaces...>
+	}
 
 	# TLS Options
 	auto_https off|disable_redirects
@@ -96,6 +103,17 @@ Customizes the [admin API endpoint](/docs/api). If `off`, then the admin endpoin
 
 - **enforce_origin** enables enforcement of the Origin header. (This is different from enforcing origins generally, which is always done.)
 
+##### `log`
+Customizes the named logger. The name can be passed to indicate a specific
+logger to customize the behavior for. If no name is specified, the behavior of
+the default logger is modified. You can read more about the default logger and
+other logging behaviors in the [logging documentation](/docs/logging/).
+
+- **output** configures where to write the logs. See the [log directive](/docs/caddyfile/directives/log#output-modules) documentation for more information, which has the same structure.
+- **format** describes how to encode, or format, the logs. See the [log directive](/docs/caddyfile/directives/log#format-modules) documentation for more information, which has the same structure.
+- **level** is the minimum entry level to log. Default: `INFO`
+- **include** identifies the loggers that are included in this log configuration. See the [JSON documentation](/docs/json/logging/logs/include/) for more information.
+- **exclude** identifies the loggers that are excluded from this log configuration. See the [JSON documentation](/docs/json/logging/logs/exclude/) for more information.
 
 
 ## TLS Options