docs: futex: Fix kernel-doc references

Since the futex code was restructured, there's no futex.c file anymore
and the implementation is split in various files. Point kernel-doc
references to the new files.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211012135549.14451-1-andrealmeid@collabora.com