docs/hw-vuln: Update desc of best effort mode

Moves the description of the best effort mitigation mode to the table of
the possible values in the mds and tsx_async_abort docs, and adds the
same one to the mmio_stale_data doc.

Signed-off-by: Takahiro Itazuri <itazur@amazon.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230901082959.28310-1-itazur@amazon.com