docs-xml: add "smbd max async dosmode"

docs-xml: add "smbd max async dosmode"

The parameter is added to the lists of ignored-paremteres in the
samba.docs tests, as the given default "aio max threads * 2" works only
as manpage string.

"aio max threads" can only be calculated at run time and requires a
handle to a pthreadpool_tevent which loadparm will never have.

Because of that lp_smbd_max_async_dosmode() will always return 0 as
default and it's up to the caller to calculate "aio max threads * 2" if
lp_smbd_max_async_dosmode() returns 0. Cf the next commit.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>