list model: Expand items-changed docs

Point out explicitly that positions change
when ::items-changed is emitted.
11 jobs for listmodel-docs in 19 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #333682
cross-android_api21_arm64

00:02:06

passed #333683
cross-android_api28_arm64

00:01:15

passed #333684
cross-mingw64

00:01:43

passed #333680
debian-stable-x86_64

00:02:06

passed #333679
fedora-x86_64

00:05:51

passed #333687
freebsd-11
freebsd-11-x86_64

00:04:01

passed #333681
G_DISABLE_ASSERT

00:03:46

passed #333685
win32
msys2-mingw32

00:13:48

passed #333686
win32
vs2017-x64

00:09:18

 
  Coverage
passed #333688
coverage

00:01:13

77.8%
 
  Analysis
passed #333689
scan-build

00:04:13