package recipe fails to build with dh_installdocs: error: Cannot find (any matches for) "/usr/doc/lablgtk3/CHANGES.md" (tried in .)

[Jason Gross]

I'm trying to backport the focal version of this package (3.1.1) (with a relaxed debhelper dependency) to focal, but the build log at https://launchpadlibrarian.net/498769572/buildlog_ubuntu-focal-amd64.lablgtk3_3.1.1-1build2.1~focal~ppa11_BUILDING.txt.gz indicates a failure in the dh_installdocs recipe with
```
[...]
# install into liblablgtk3-ocaml-dev all regular files of the
# lablgtk3 dir that are not to be installed into liblablgtk3-ocaml
# Same for liblablgtksourceview3 and liblablgtkspell3
(cd /<<PKGBUILDDIR>>/debian/tmp; \
 find usr/lib/ocaml/lablgtk3 \
   -type f \
   ! -exec grep -q '{}' ../liblablgtk3-ocaml.install \;\
   -print \
   >> ../liblablgtk3-ocaml-dev.install; \
 find usr/lib/ocaml/lablgtk3-sourceview3 \
   -type f \
   ! -exec grep -q '{}' ../liblablgtksourceview3-ocaml.install \;\
   -print \
   >> ../liblablgtksourceview3-ocaml-dev.install; \
 find usr/lib/ocaml/lablgtk3-gtkspell3 \
   -type f \
   ! -exec grep -q '{}' ../liblablgtkspell3-ocaml.install \;\
   -print \
   >> ../liblablgtkspell3-ocaml-dev.install)
make[1]: Leaving directory '/<<PKGBUILDDIR>>'
   dh_install
   dh_ocamldoc
   dh_installdocs
dh_installdocs: error: Cannot find (any matches for) "/usr/doc/lablgtk3/CHANGES.md" (tried in .)

make: *** [debian/rules:9: binary] Error 25
dpkg-buildpackage: error: debian/rules binary subprocess returned exit status 2
```
If this is an issue with the version of debhelper, then feel free to close this, but otherwise it seems to indicate an issue in the rules file