docs: document .torchxconfig usage with component: (#434)

Summary:
This documents `.torchxconfig` `[component:foo]` defaults.

Pull Request resolved: #434

Test Plan: ![Screenshot 2022-03-25 at 15-53-37  torchxconfig — PyTorch TorchX main documentation](https://user-images.githubusercontent.com/909104/160212166-b6073c1d-2afd-4186-b1ed-4304e1e08248.png)

Reviewed By: kiukchung

Differential Revision: D35159963

Pulled By: d4l3k

fbshipit-source-id: 6f5b8c912726c29ac02e373e53b1e708e868bd08

Author: d4l3k

torchx/runner/config.py

@@ -15,6 +15,9 @@
 CLI Usage
 ~~~~~~~~~~~
 
+
+**Scheduler Config**
+
 #. ``cd`` into the directory where you want the ``.torchxconfig`` file to be dropped.
    The CLI only picks up ``.torchxconfig`` files from the current-working-directory (CWD)
    so chose a directory where you typically run ``torchx`` from. Typically this
@@ -66,6 +69,43 @@
 
     $ torchx run -s local_cwd ./my_component.py:train
 
+**Component Config**
+
+You can specify component defaults by adding a section prefixed with
+``component:``.
+
+.. code-block:: ini
+
+    [component:dist.ddp]
+    j=2x8
+    cpu=4
+
+Now when you run the ``dist.ddp`` component those configs are automatically
+picked up.
+
+.. code-block:: shell-session
+
+    $ torchx run -s local_cwd dist.ddp
+    ... runs with -j 2x8 --cpu 4
+
+
+**CLI Subcommand Config**
+
+The default arguments for the ``torchx`` subcommands can be overwritten. Any
+``--foo FOO`` argument can be set via the correspond ``[cli:<cmd>]`` settings
+block.
+
+For the ``run`` command you can additionally set ``component`` to set the
+default component to run.
+
+.. code-block:: ini
+
+    [cli:run]
+    component=dist.ddp
+    scheduler=local_docker
+    workspace=file://some_workspace
+
+
 Programmatic Usage
 ~~~~~~~~~~~~~~~~~~~
 
@@ -265,7 +305,7 @@ def load_sections(
      j = 1x2
      image = ghcr.io/foo:1
 
-     # calling `load_component_defaults(prefix="component")` returns
+     # calling `load_sections(prefix="component")` returns
      #  {
      #    "dist.ddp": {
      #       "j":"1x2",