trace: Update documentation

Revamp the documentation for the new features, including a description of
the new features and documentation for the trace command.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 13e6939..cde7dcb 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -84,6 +84,7 @@
    cmd/sm
    cmd/temperature
    cmd/tftpput
+   cmd/trace
    cmd/true
    cmd/ums
    cmd/ut