perf report: Add latency and parallelism profiling documentation

Describe latency and parallelism profiling, related flags, and differences
with the currently only supported CPU-consumption-centric profiling.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Link: https://lore.kernel.org/r/a13f270ed33cedb03ce9ebf9ddbd064854ca0f19.1739437531.git.dvyukov@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>