Overlay: Document overlay children aren't measured

Some users expect that the Overlay will automatically request enough
size for its overlay children as well as its main child. It doesn't,
because it's just a GtkBin. Add a short paragraph pointing that out.

Close #1939
4 jobs for wip/dboles/overlay-docs-3 in 18 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #325569
fedora-autotools

00:10:21

failed #325570
fedora-meson

00:03:25

manual #325572
win32 allowed to fail manual
msys2-mingw32-autotools
passed #325571
win32
msys2-mingw32-meson

00:18:41

 
Name Stage Failure
failed
fedora-meson Build
                             bg-full-height: 0
direction: <omitted>
-------

Full log written to /builds/GNOME/gtk/_build/meson-logs/testlog.txt
Uploading artifacts...
_build/meson-logs: found 4 matching files
Uploading artifacts to coordinator... ok id=325570 responseStatus=201 Created token=8EztSHAg
ERROR: Job failed: exit code 1