docs: correct the shaderc documentation
docs: correct the shaderc documentation

The documentation on how shaderc is checked in meson was quite behind.
Update it to mention that pkg-config is the default and preferred method
of checking. Also be specific about what order everything is checked in
since shaderc is confusing.

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