Documentation: futex: Add a note about robust list race condition

Documentation: futex: Add a note about robust list race condition

Add a note to the documentation giving a brief explanation why doing a
robust futex release in userspace is racy, what should be done to avoid
it and provide links to read more.

[ tglx: Fixed a few typos ]

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260329-tonyk-vdso_test-v2-1-b7db810e44a1@igalia.com
Link: https://patch.msgid.link/20260602090535.936286833@kernel.org