doc: Improve documentation for the bootm command

Reformat and rewrite the documentation for this command.

This is a complicated command, so further improvements are welcome.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 04c959a..29ae8a1 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -32,6 +32,7 @@
    cmd/bootefi
    cmd/bootflow
    cmd/booti
+   cmd/bootm
    cmd/bootmenu
    cmd/bootmeth
    cmd/button