'bind' default value documentation

I'm not sure if there is a way to directly contribute to the documentation, but hopefully this gets where it needs to go.

During my first use of `sinatra` today I had an issue of getting up and running just as easily as the documentation seems to imply. After a little digging it was because my environment was being default-ed to `development`. This may be common or expected knowledge in the ruby community, but being less experienced in the language caused a small pitfall.

Thanks for your time, and sorry if I'm way off base on my request!

Relevent code:
https://github.com/sinatra/sinatra/blob/master/lib/sinatra/base.rb#L1734
https://github.com/sinatra/sinatra/blob/master/lib/sinatra/base.rb#L1765