Documentation: userspace-api: Document perf ring buffer mechanism

In the Linux perf tool, the ring buffer serves not only as a medium for
transferring PMU event data but also as a vital mechanism for hardware
tracing using technologies like Intel PT and Arm CoreSight, etc.

Consequently, the ring buffer mechanism plays a crucial role by ensuring
high throughput for data transfer between the kernel and user space
while avoiding excessive overhead caused by the ring buffer itself.

This commit documents the ring buffer mechanism in detail.  It explains
the implementation of both the regular ring buffer and the AUX ring
buffer.  Additionally, it covers how these ring buffers support various
tracing modes and explains the synchronization with memory barriers.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: James Clark <james.clark@arm.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240102085001.228815-1-leo.yan@linaro.org