docs: Add 'cygwin' to reference table for .system()
docs: Add 'cygwin' to reference table for .system()

This aligns this table with the set of values tested for by test case
common/140

becm/meson,QuLogic/meson,thiblahute/meson,trhd/meson,thiblahute/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,trhd/meson,mesonbuild/meson,ernestask/meson,becm/meson,jpakkane/meson,jeandet/meson,mesonbuild/meson,pexip/meson,MathieuDuponchelle/meson,ernestask/meson,jpakkane/meson,becm/meson,becm/meson,jeandet/meson,becm/meson,MathieuDuponchelle/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,ernestask/meson,ernestask/meson,becm/meson,ernestask/meson,pexip/meson,mesonbuild/meson,jeandet/meson,jeandet/meson,ernestask/meson,MathieuDuponchelle/meson,thiblahute/meson,trhd/meson,trhd/meson,ernestask/meson,MathieuDuponchelle/meson,trhd/meson,QuLogic/meson,jeandet/meson,trhd/meson,thiblahute/meson,QuLogic/meson,becm/meson,pexip/meson,mesonbuild/meson,trhd/meson,thiblahute/meson,jpakkane/meson,pexip/meson,thiblahute/meson,QuLogic/meson,MathieuDuponchelle/meson,jpakkane/meson,pexip/meson,jpakkane/meson,trhd/meson,thiblahute/meson,MathieuDuponchelle/meson,QuLogic/meson,trhd/meson,mesonbuild/meson,ernestask/meson,QuLogic/meson,jeandet/meson,MathieuDuponchelle/meson,ernestask/meson,becm/meson,pexip/meson,pexip/meson,QuLogic/meson,pexip/meson,MathieuDuponchelle/meson,QuLogic/meson,jeandet/meson,mesonbuild/meson,becm/meson,thiblahute/meson,jpakkane/meson,thiblahute/meson,MathieuDuponchelle/meson,jeandet/meson,jpakkane/meson,jeandet/meson,becm/meson,pexip/meson