[Docs] `--help`: explain more clearly that <version> parameter is optional and can be provided by .nvmrc on `nvm install`, `nvm use`, `nvm exec`, and `nvm run`.
[Docs] `--help`: explain more clearly that <version> parameter is optional and can be provided by .nvmrc on `nvm install`, `nvm use`, `nvm exec`, and `nvm run`.

creationix/nvm,ljharb/nvm,creationix/nvm,PeterDaveHello/nvm,ljharb/nvm,PeterDaveHello/nvm