docs/perf: Add explanation for DDR_CAP_AXI_ID_FILTER_ENHANCED quirk

Add explanation for DDR_CAP_AXI_ID_FILTER_ENHANCED quirk.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
[will: Simplified wording]
Signed-off-by: Will Deacon <will@kernel.org>