vfs_aio_pthread.8.xml: Update per discussion on samba mailing list

[github at samba.org]

New comment by urisimchoni on Samba Github repository

https://github.com/samba-team/samba/pull/144#issuecomment-446867892
Comment:
The pthread-based AIO is now the default in Samba (not via a module), and vfs_aio_pthread.8.xml is only about async open. The changes proposed here are already incorporated in the main smb.conf file.