Documentation: checkpatch: Tweak BIT() macro include

While include/linux/bitops.h brings in the BIT() macro, it was moved to
include/linux/bits.h in commit 8bd9cb51 ("locking/atomics, asm-generic:
Move some macros from <linux/bitops.h> to a new <linux/bits.h> file").

Since that commit BIT() has moved again into include/vdso/bits.h via
commit 3945ff37

 ("linux/bits.h: Extract common header for vDSO").

I think the move to the vDSO header can be considered an implementation
detail, so for now update the checkpatch documentation to recommend use
of include/linux/bits.h.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Jiri Slaby <jirislaby@kernel.org>
Acked-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20210520093949.511471-1-andrew@aj.id.au
Signed-off-by: Jonathan Corbet <corbet@lwn.net>