Auto merge of #4031 - Zorbash:binstubs-description, r=segiddins
Auto merge of #4031 - Zorbash:binstubs-description, r=segiddins

Update binstubs command description for multiple gems

The current long description or the `binstubs` command does not provide
any hint that it can accept multiple gems as arguments and generate
binstubs for all of them.

This PR enhances the description using similar phrasing for usage
with multiple gem arguments, to the `outdated` command.

jingweno/bundler,dilumnavanjana/bundler,asutoshpalai/bundler,RochesterinNYC/bundler,pivotal-cf-experimental/bundler,Teino1978-Corp/Teino1978-Corp-bundler,bronzdoc/bundler,mpapis/bundler,RochesterinNYC/bundler,dilumnavanjana/bundler,bundler/bundler,mvz/bundler,davydovanton/bundler,mpapis/bundler,bundler/bundler,davydovanton/bundler,seanlinsley/bundler,bronzdoc/bundler,mvz/bundler,Teino1978-Corp/Teino1978-Corp-bundler,seanlinsley/bundler,pivotal-cf-experimental/bundler,jingweno/bundler,joshsoftware/bundler,joshsoftware/bundler,asutoshpalai/bundler