docs: fix the docs for install_subdir's strip_directory [skip ci]
docs: fix the docs for install_subdir's strip_directory [skip ci]

When set to **true** only the last component is used. And throw in two minor
grammatical fixes while we're there.
mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,becm/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,mesonbuild/meson,becm/meson,becm/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson,QuLogic/meson,becm/meson,jpakkane/meson,pexip/meson,QuLogic/meson,becm/meson,pexip/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,pexip/meson,becm/meson,mesonbuild/meson,becm/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,becm/meson,becm/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,pexip/meson,becm/meson,QuLogic/meson