linux/types.h: reinstate "__bitwise__" macro for user space use

Commit c724c866

 ("linux/types.h: remove unnecessary __bitwise__")
was right that there are no users of __bitwise__ in the kernel, but it
turns out there are user space users of it that do expect it.

It is, after all, in the uapi directory, so user space usage is to be
expected.

Instead of reverting the commit completely, let's just clarify the
situation so that it doesn't happen again, and have some in-code
explanations for why that "__bitwise__" still exists.

Reported-by: Jiri Slaby <jirislaby@kernel.org>
Cc: Bjorn Helgaas <helgaas@kernel.org>
Link: https://lore.kernel.org/all/b5c0a68d-8387-4909-beea-f70ab9e6e3d5@kernel.org/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>