docs: fix description of default_options
docs: fix description of default_options

For dependency() and subproject(), the default_options argument can
override the defaults in the subproject's meson_options.txt, not in
its project() invocation.

Resolves #2612

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