[tune] Add documentation to --output flag (ray-project#4518)

## What do these changes do?

Add documentation for the `--output` flag for ls / lsx in the Tune CLI.

## Related issue number

Closes ray-project#4511 

## Linter

- [x] I've run `scripts/format.sh` to lint the changes in this PR.

doc/source/tune-usage.rst

@@ -465,11 +465,11 @@ Tune CLI (Experimental)
 
 Here are a few examples of command line calls.
 
-- ``tune list-trials``: List tabular information about trials within an experiment. Add the ``--sort`` flag to sort the output by specific columns. Add the ``--filter`` flag to filter the output in the format ``"<column> <operator> <value>"``.
+- ``tune list-trials``: List tabular information about trials within an experiment. Empty columns will be dropped by default. Add the ``--sort`` flag to sort the output by specific columns. Add the ``--filter`` flag to filter the output in the format ``"<column> <operator> <value>"``. Add the ``--output`` flag to write the trial information to a specific file (CSV or Pickle).
 
 .. code-block:: bash
 
-    $ tune list-trials [EXPERIMENT_DIR]
+    $ tune list-trials [EXPERIMENT_DIR] --output note.csv
 
     +------------------+-----------------------+------------+
     | trainable_name   | experiment_tag        | trial_id   |
@@ -481,6 +481,8 @@ Here are a few examples of command line calls.
     | MyTrainableClass | 4_height=90,width=69  | ae4e02fb   |
     +------------------+-----------------------+------------+
     Dropped columns: ['status', 'last_update_time']
+    Please increase your terminal size to view remaining columns.
+    Output saved at: note.csv
 
     $ tune list-trials [EXPERIMENT_DIR] --filter "trial_id == 7b99a28a"
 
@@ -490,12 +492,13 @@ Here are a few examples of command line calls.
     | MyTrainableClass | 3_height=54,width=21  | 7b99a28a   |
     +------------------+-----------------------+------------+
     Dropped columns: ['status', 'last_update_time']
+    Please increase your terminal size to view remaining columns.
 
-- ``tune list-experiments``: List tabular information about experiments within a project. Add the ``--sort`` flag to sort the output by specific columns. Add the ``--filter`` flag to filter the output in the format ``"<column> <operator> <value>"``.
+- ``tune list-experiments``: List tabular information about experiments within a project. Empty columns will be dropped by default. Add the ``--sort`` flag to sort the output by specific columns. Add the ``--filter`` flag to filter the output in the format ``"<column> <operator> <value>"``. Add the ``--output`` flag to write the trial information to a specific file (CSV or Pickle).
 
 .. code-block:: bash
 
-    $ tune list-experiments [PROJECT_DIR]
+    $ tune list-experiments [PROJECT_DIR] --output note.csv
 
     +----------------------+----------------+------------------+---------------------+
     | name                 |   total_trials |   running_trials |   terminated_trials |
@@ -505,6 +508,8 @@ Here are a few examples of command line calls.
     | hyperband_test       |              1 |                0 |                   1 |
     +----------------------+----------------+------------------+---------------------+
     Dropped columns: ['error_trials', 'last_updated']
+    Please increase your terminal size to view remaining columns.
+    Output saved at: note.csv
 
     $ tune list-experiments [PROJECT_DIR] --filter "total_trials <= 1" --sort name
 
@@ -515,6 +520,7 @@ Here are a few examples of command line calls.
     | test                 |              1 |                0 |                   0 |
     +----------------------+----------------+------------------+---------------------+
     Dropped columns: ['error_trials', 'last_updated']
+    Please increase your terminal size to view remaining columns.
 
 
 Further Questions or Issues?

python/ray/tune/commands.py

@@ -194,16 +194,14 @@ def list_trials(experiment_path,
     print_format_output(checkpoints_df)
 
     if output:
-        experiment_path = os.path.expanduser(experiment_path)
-        output_path = os.path.join(experiment_path, output)
         file_extension = os.path.splitext(output)[1].lower()
         if file_extension in (".p", ".pkl", ".pickle"):
-            checkpoints_df.to_pickle(output_path)
+            checkpoints_df.to_pickle(output)
         elif file_extension == ".csv":
-            checkpoints_df.to_csv(output_path, index=False)
+            checkpoints_df.to_csv(output, index=False)
         else:
             raise ValueError("Unsupported filetype: {}".format(output))
-        print("Output saved at:", output_path)
+        print("Output saved at:", output)
 
 
 def list_experiments(project_path,
@@ -295,15 +293,14 @@ def list_experiments(project_path,
     print_format_output(info_df)
 
     if output:
-        output_path = os.path.join(base, output)
         file_extension = os.path.splitext(output)[1].lower()
         if file_extension in (".p", ".pkl", ".pickle"):
-            info_df.to_pickle(output_path)
+            info_df.to_pickle(output)
         elif file_extension == ".csv":
-            info_df.to_csv(output_path, index=False)
+            info_df.to_csv(output, index=False)
         else:
             raise ValueError("Unsupported filetype: {}".format(output))
-        print("Output saved at:", output_path)
+        print("Output saved at:", output)
 
 
 def add_note(path, filename="note.txt"):