docs: document config file parameter on CLI page

The CLI page had the config file parameter missing. This commit
adds that. Also adds the CLI usage output which shows all
available options. Closes #4300.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

docs/references/cli.rst

@@ -3,23 +3,45 @@
 CLI
 ===
 
-PostgREST provides a CLI with the commands listed below:
+PostgREST provides a CLI with the options listed below:
+
+.. code:: text
+
+  Usage: postgrest [-v|--version] [-e|--example] [--dump-config | --dump-schema] 
+                 [FILENAME]
+
+    PostgREST / create a REST API to an existing Postgres
+    database
+
+  Available options:
+    -h,--help                Show this help text
+    -v,--version             Show the version information
+    -e,--example             Show an example configuration file
+    --dump-config            Dump loaded configuration and exit
+    --dump-schema            Dump loaded schema as JSON and exit (for debugging,
+                             output structure is unstable)
+    FILENAME                 Path to configuration file
+
+FILENAME
+--------
+
+Runs PostgREST with the given :ref:`file_config`.
 
 Help
 ----
 
 .. code:: bash
 
-  $ postgrest [-h|--help]
+  $ postgrest --help
 
-Shows all the commands available.
+Shows all the options available.
 
 Version
 -------
 
 .. code:: bash
 
-  $ postgrest [-v|--version]
+  $ postgrest --version
 
 Prints the PostgREST version.
 
@@ -28,16 +50,16 @@ Example
 
 .. code:: bash
 
-  $ postgrest [-e|--example]
+  $ postgrest --example
 
-Shows example configuration options.
+Shows example configuration settings.
 
 Dump Config
 -----------
 
 .. code:: bash
 
-  $ postgrest [--dump-config]
+  $ postgrest --dump-config
 
 Dumps the loaded :ref:`configuration` values, considering the configuration file, environment variables and :ref:`in_db_config`.
 
@@ -46,6 +68,6 @@ Dump Schema
 
 .. code:: bash
 
-  $ postgrest [--dump-schema]
+  $ postgrest --dump-schema
 
 Dumps the schema cache in JSON format.