Document correct filesystem config options in reference (#4928)

Closes #4915

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

docs/sources/configuration/_index.md

@@ -2387,12 +2387,25 @@ If any specific configuration for an object storage client have been provided el
 [swift: <swift_storage_config>]
 
 # Configures a (local) filesystem as the common storage.
-[filesystem: <local_storage_config>]
+[filesystem: <fs_storage_config>]
 
 # The `hedging_config` configures how to hedge requests for the storage.
 [hedging: <hedging_config>]
 ```
 
+### fs_storage_config
+
+The `fs_storage_config` configures a local filesystem as a general
+storage for different data generated by Loki.
+
+```yaml
+# Filesystem directory to be used for chunks storage.
+[chunks_directory: <filename> | default = ""]
+
+# Filesystem directory to be used for rules storage.
+[rules_directory: <filename> | default = ""]
+```
+
 ### ring_config
 
 The `ring_config` blocks defines a ring configuration used by Loki component.