Document effect of a bugfix on CLS

The bugfix is:
https://chromium-review.googlesource.com/c/chromium/src/+/2586969

Bug: 1141739

Change-Id: I8056b71d60442f53ab52d62066e152f76ed02db0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2615305Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841175}

docs/speed/metrics_changelog/2020_11_cls.md

@@ -61,6 +61,13 @@ their scores.
 Sites using visibility:hidden to hide layout changes may see a decrease in
 their Cumulative Layout Shift scores.
 
+### Fixed bug when recording paint offset translation deltas
+
+A bug was introduced in Chrome 86 in which Cumulative Layout Shift was measured
+incorrectly for certain cases involving nested out-of-flow elements
+(absolute-position or fixed-position). This caused reported shifts even in
+some cases where there was not actually a shift.
+
 ## When were users affected?
 
 Chrome 88 is currently scheduled to be released the week of January 19, 2021.