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.auSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/dev-tools/checkpatch.rst

@@ -512,7 +512,7 @@ Macros, Attributes and Symbols
 
   **BIT_MACRO**
     Defines like: 1 << <digit> could be BIT(digit).
-    The BIT() macro is defined in include/linux/bitops.h::
+    The BIT() macro is defined via include/linux/bits.h::
 
       #define BIT(nr)         (1UL << (nr))