docs: flash_update.md: tweaks and cleanup

Some overall cleanup and tweaks to style.  The implementation now
supports registration under the Firmware netfn.  The extra 3-bytes for
the OEM will drop from this document when it's fully transitioned.

Some of the changes were missed during the initial check-in.

Change-Id: I255114f62e1ffdebde2412ecdfc6a1acb7ff1b1e
Signed-off-by: Patrick Venture <venture@google.com>
1 file changed