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Help: Add 3.20 release note for error on unknown arguments

This was left out of commit 1b6c5333 (cmake: Error out on unknown
arguments starting with `-`., 2020-11-30, v3.20.0-rc1~370^2).

Fixes: #22060
29 jobs for !6003 with doc-error-unknown-arg in 6 minutes and 33 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Build
manual build:centos6-x86_64 #5703736
cmake build docker linux manual
passed build:debian10-iwyu #5703733
cmake build docker linux

00:04:30

manual build:fedora33-ninja #5703737
cmake build docker linux linux-3.17 manual
passed build:fedora33-sphinx #5703735
cmake build docker linux linux-3.17

00:04:46

passed build:fedora33-tidy #5703734
cmake build docker linux linux-3.17

00:06:33

manual build:macos-x86_64-makefiles #5703739
cmake macos shell xcode-12.4 nonconcurrent manual
manual build:macos-x86_64-ninja #5703738
cmake macos shell xcode-12.4 nonconcurrent manual
manual build:windows-vs2019-x64-ninja #5703740
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent manual
 
  Test
created test:fedora33-ninja #5703741
cmake docker linux linux-3.17 x11
created test:macos-x86_64-makefiles #5703743
cmake macos shell xcode-12.4 nonconcurrent
created test:macos-x86_64-ninja #5703742
shell xcode-12.4 nonconcurrent cmake macos
created test:windows-vs2019-x64-ninja #5703744
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent
 
  Test Ext
created test:cuda10.2-nvidia #5703747
cmake cuda-rt docker linux
created test:debian10-ninja #5703745
cmake build docker linux linux-3.17
created test:fedora33-makefiles #5703746
cmake build docker linux linux-3.17
created test:fedora33-ninja-multi #5703748
cmake build docker linux linux-3.17
created test:macos-x86_64-xcode #5703749
cmake macos shell xcode-12.4 concurrent
created test:windows-vs2019-x64 #5703750
cmake windows shell vs2019 msvc-19.28-16.9 concurrent
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager