dm: Add documentation for host command and implementation

Document the 'host' command and also the internals of how it is
implemented.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index f3c8fba..f7f03ae 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -52,6 +52,7 @@
    cmd/for
    cmd/fwu_mdata
    cmd/gpio
+   cmd/host
    cmd/load
    cmd/loadm
    cmd/loady