Documentation: update notes in biovecs about arbitrarily sized bios

Update block/biovecs.txt so that it includes a note on what kind of
effects arbitrarily sized bios would bring to the block layer.
Also fix a trivial typo, bio_iter_iovec.

Signed-off-by: Dongsu Park <dongsu.park@profitbricks.com>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org