Documentation/arm64/tagged-address-abi.rst introduces the
relaxation of the syscall ABI that allows userspace to pass
certain tagged pointers to kernel syscalls.

Add the document to index.rst for a correct generation of the
table of content.

Cc: Will Deacon
Cc: Catalin Marinas
Signed-off-by: Vincenzo Frascino
Signed-off-by: Will Deacon