scylla-nodetool, docs: improve optional params documentation

Document the behavior if no keyspace is specified
or no table(s) are specified for a given keyspace.

Fixes scylladb/scylladb#16032

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
(cherry picked from commit 9324363e55)

docs/operating-scylla/nodetool-commands/flush.rst

@@ -1,6 +1,6 @@
 Nodetool flush
 ==============
-**flush** ``[<keyspace> <cfnames>...]``- Specify a keyspace and one or more tables that you want to flush from the memtable to on disk SSTables.
+**flush** - Flush memtables to on-disk SSTables in the specified keyspace and table(s).
 
 For example:
 
@@ -8,6 +8,26 @@ For example:
 
    nodetool flush keyspaces1 standard1
 
+Syntax
+------
+
+.. code-block:: none
+
+   nodetool flush [<keyspace> [<table> ...]]
+
+nodetool flush takes the following parameters:
+
+.. list-table::
+   :widths: 50 50
+   :header-rows: 1
+
+   * - Parameter Name
+     - Description
+   * - ``<keyspace>``
+     - The keyspace to operate on.  If omitted, all keyspaces are flushed.
+   * - ``<table> ...``
+     - One or more tables to operate on.  Tables may be specified only if a keyspace is given.  If omitted, all tables in the specified keyspace are flushed.
+
 See also
 
 :doc:`Nodetool drain </operating-scylla/nodetool-commands/drain/>`