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/arch/index.rst b/doc/arch/index.rst
index 792d918..b3e85f9 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -11,7 +11,7 @@
    m68k
    mips
    nios2
-   sandbox
+   sandbox/index
    sh
    x86
    xtensa