stat.2, statx.2: Fix descriptions for AT_NO_AUTOMOUNT

stat.2, statx.2: Fix descriptions for AT_NO_AUTOMOUNT

fstatat(..., AT_NO_AUTOMOUNT) has had the following history in Linux:
  v2.6.37-7314-g6f45b65672c8
    add AT_NO_AUTOMOUNT and fstatat honors it
  v3.1-rc7-68-gb6c8069d3577
    ignore AT_NO_AUTOMOUNT since default operation now less eagerly mounts
  v4.10-11255-ga528d35e8bfc
    adds statx which reinstated 2.6.38 behavior for fstatat (not released)
  v4.11-rc7-14-gdeccf497d804
    adjust fstatat so that AT_NO_AUTOMOUNT always specified (to statx)

* man2/stat.2:
Adjust AT_NO_AUTOMOUNT description for fstatat.2 as per the above,
to indicate AT_NO_AUTOMOUNT should be avoided with fstatat() since
it's ignored since 3.1 and implied since 4.11.

Don't mention commit v4.13-9318-g42f461482178 as it was reverted,
and moot anyway since we can't adjust AT_NO_AUTOMOUNT since 3.1.

* man2/statx.2:
Mention that stat(), lstat(), and fstatat() imply AT_NO_AUTOMOUNT.

Signed-off-by: Pádraig Brady <P@draigBrady.com>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>