system_config: add path attribute in system/log section

deployment/logging.md

@@ -148,13 +148,27 @@ end
 
 ## Output to Log File
 
+### By Command Line Option
+
 By default, Fluentd outputs to the standard output. Use `-o` command line option to specify the file instead:
 
 ```text
 $ fluentd -o /path/to/log_file
 ```
 
-### Log Rotation Setting
+### By Config File
+
+Since v1.18.0, You can also configure the log file path using the `<log>` directive under `<system>`:
+
+```text
+<system>
+  <log>
+    path /path/to/log_file
+  </log>
+</system>
+```
+
+## Log Rotation Setting
 
 By default, Fluentd does not rotate log files. You can configure this behavior via system-config after v1.13.0.
 
@@ -176,7 +190,7 @@ Actually, an external library manages these default values, resulting in this co
 
 You can use command-line options too (mainly for before v1.13.0):
 
-#### `--log-rotate-age AGE`
+### `--log-rotate-age AGE`
 
 `AGE` is an integer or a string:
 
@@ -185,7 +199,7 @@ You can use command-line options too (mainly for before v1.13.0):
 
 NOTE: When `--log-rotate-age` is specified on Windows, log files are separated into `log-supervisor-0.log`, `log-0.log`, ..., `log-N.log` where `N` is `generation - 1` due to the system limitation. Windows does not permit delete and rename files simultaneously owned by another process.
 
-#### `--log-rotate-size BYTES`
+### `--log-rotate-size BYTES`
 
 The byte size to rotate log files. This is applied when `--log-rotate-age` is specified with integer:
 

deployment/system-config.md

@@ -174,6 +174,16 @@ Enable JIT for worker processes. Internally, this configuration enables Ruby's `
 
 ### `<log>` section
 
+#### `path`
+
+| type | default | version |
+| :--- | :--- | :--- |
+| string | nil | 1.18.0 |
+
+Specifies the log file path.
+
+Please see also [Output to Log File](logging.md#output-to-log-file).
+
 #### `format`
 
 | type | default | version |