docs: document default version for declare_dependency()
docs: document default version for declare_dependency()

By default, the project version is picked.

jpakkane/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,pexip/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson