hmp: Fix texinfo documentation

hmp: Fix texinfo documentation

Several issues:

* Commands i and o lack @item.  Their one-liner documentation gets
  squashed into the preceding command print.  Add the obvious @item.

* Commands i, o and cpu-add lack @findex.  The function index doesn't
  have them.  Add the obvious @findex.

* Commit 727f005 put block_set_io_throttle was added in the middle of
  block_passwd.  Move it.

* Correct spelling of commands chardev-add and chardev-remove in @item
  and @findex.

* Some commands have a blank line between @item/@findex and the text,
  most don't.  Normalize to no blank line.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>