docs: admin-guide: Remove threads-max auto-tuning

Since following path was merged in 5.4-rc3,
auto-tuning feature in threads-max does not exist any more.
Fix the admin-guide document as is.

kernel/sysctl.c: do not override max_threads provided by userspace
b0f53dbc

Fixes: b0f53dbc ("kernel/sysctl.c: do not override max_threads provided by userspace")
Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/sysctl/kernel.rst

@@ -1111,10 +1111,6 @@ constant FUTEX_TID_MASK (0x3fffffff).
 If a value outside of this range is written to threads-max an error
 EINVAL occurs.
 
-The value written is checked against the available RAM pages. If the
-thread structures would occupy too much (more than 1/8th) of the
-available RAM pages threads-max is reduced accordingly.
-
 
 unknown_nmi_panic:
 ==================