Merge 'docs: document scylla-api-client' from Anna Stuchlik

Fixes #11999.

This PR adds a description of scylla-api-cli.

Closes #12392

* github.com:scylladb/scylladb:
  docs: fix the description of the system log POST example
  docs: uptate the curl tool name
  docs: describe how to use the scylla-api-client tool
  docs: fix the scylla-api-client tool name
  docs: document scylla-api-cli

docs/operating-scylla/rest.rst

@@ -2,7 +2,35 @@
 REST
 ----
 
-For each JMX operation, attribute get and set, Scylla exposes a matching REST API. You can interact with the REST API using :code:`curl` or using the Swagger UI available at :code:`your-ip:10000/ui`
+ScyllaDB exposes a REST API to retrieve administrative information from a node and execute 
+administrative operations. For example, it allows you to check or update configuration, 
+retrieve cluster-level information, and more.
 
 
+CLI for REST API
+^^^^^^^^^^^^^^^^^
 
+ScyllaDB ships with ``scylla-api-client``, a lightweight tool that provides a command-line interface 
+for the ScyllaDB API. The tool allows you to:
+
+* List the API functions with their parameters and print detailed help for each function.
+* Invoke the API commands using the command line. 
+
+When invoking a function, ``scylla-api-client`` performs basic validation of the function arguments and 
+prints the result to the standard output. You can use commonly available command line utilities
+to print the results in the JSON format.
+
+To avoid errors, you should prefer ``scylla-api-client`` over ``curl`` and similar HTTP tools for interacting 
+with the ScyllaDB REST API.
+
+Usage
+======
+
+Run ``scylla-api-client --help`` for information about all the available options.
+
+**Examples:**
+
+* ``scylla-api-client --list-modules`` - Shows all the API modules.
+* ``scylla-api-client --list-module-commands system`` - Shows all the API commands for the ``system`` module.
+* ``scylla-api-client system uptime_ms`` - Gets the system uptime (in milliseconds).
+* ``scylla-api-client system log POST --message "hello world" --level warn`` - Writes the "hello world" message to the ScyllaDB log using the "warn" logging level.