docs: add missing docs for new pkgconfig feature
docs: add missing docs for new pkgconfig feature

Added in commit 6240920c213fb76b4e4be8b6b59ae3346cbbcf77 and documented
in release notes but not in module docs.

mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson