[doc] updated docs for rebuilding native modules
[doc] updated docs for rebuilding native modules

Use `npm_config_` environments to override default behaviors of
npm and node-gyp. `npm_config_node_gyp` can be used to replace
`node-gyp` with `nw-gyp` during `npm install`.

nwjs/nw.js,nwjs/nw.js,nwjs/nw.js,nwjs/nw.js,nwjs/nw.js,nwjs/nw.js