threads: Update PID limit comment according to futex UAPI change

The futex UAPI changed back in commit 76b81e2b ("[PATCH] lightweight
robust futexes updates 2"), which landed in v2.6.17: FUTEX_TID_MASK is now
0x3fffffff instead of 0x1fffffff. Update the corresponding comment in
include/linux/threads.h.

Documentation mentions that only the lower 29 bits are available for TID
storage, but as the UAPI header released the bit already via
FUTEX_TID_MASK, this is moot as well. Fix it up.

[ tglx: Fixed up documentation ]

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200302112939.8068-1-jannh@google.com