docs: add `W` := `W.x` & `W.Y` := `W.Y.x`; clarify that `W.Y.Z` := `=W.Y.Z`
docs: add `W` := `W.x` & `W.Y` := `W.Y.x`; clarify that `W.Y.Z` := `=W.Y.Z`

Fixes #74.

bitovi/node-semver,stevemao/node-semver,jasonkuhrt/node-semver,ekmartin/node-semver,stealjs/node-semver,you49net/Repos,hippich/bower-semver,thenaughtychild/node-semver,megawac/node-semver,npm/node-semver,rvagg/sanever