Update Reference-manual.md
Update Reference-manual.md

Add notes about not using full_path() unless absolutely necessary.
jeandet/meson,jeandet/meson,trhd/meson,becm/meson,MathieuDuponchelle/meson,jpakkane/meson,MathieuDuponchelle/meson,jpakkane/meson,thiblahute/meson,ernestask/meson,thiblahute/meson,trhd/meson,pexip/meson,jeandet/meson,thiblahute/meson,ernestask/meson,QuLogic/meson,MathieuDuponchelle/meson,jeandet/meson,jeandet/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,MathieuDuponchelle/meson,thiblahute/meson,jpakkane/meson,trhd/meson,jeandet/meson,becm/meson,MathieuDuponchelle/meson,becm/meson,ernestask/meson,MathieuDuponchelle/meson,mesonbuild/meson,thiblahute/meson,mesonbuild/meson,ernestask/meson,jeandet/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,thiblahute/meson,QuLogic/meson,becm/meson,mesonbuild/meson,becm/meson,MathieuDuponchelle/meson,jpakkane/meson,ernestask/meson,becm/meson,becm/meson,mesonbuild/meson,trhd/meson,pexip/meson,pexip/meson,trhd/meson,QuLogic/meson,QuLogic/meson,QuLogic/meson,pexip/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,thiblahute/meson,becm/meson,pexip/meson,trhd/meson,ernestask/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,ernestask/meson,trhd/meson,ernestask/meson,jeandet/meson,becm/meson,thiblahute/meson,pexip/meson,QuLogic/meson,pexip/meson,MathieuDuponchelle/meson,jeandet/meson,mesonbuild/meson,becm/meson,trhd/meson,thiblahute/meson,pexip/meson,mesonbuild/meson,MathieuDuponchelle/meson,QuLogic/meson,trhd/meson,ernestask/meson,jpakkane/meson