docs: Explain how to use a Vala project's VAPI files with find_library() [skip ci]
docs: Explain how to use a Vala project's VAPI files with find_library() [skip ci]

The Vala compiler object's find_libary() method needs the 'dir' keyword to
add a project's local vapi dir to the VAPI search path.

See https://github.com/mesonbuild/meson/issues/4702

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