Clarify Rails uses erubis not stdlin ERB. [ci skip]

https://github.com/rails/rails/issues/16766#issuecomment-54148778 tells me Rails uses erubis, not the stdlib ERB.

In the erubis doc, I could not find it explicitly written that it is equivalent to ERB, or ERB with some options.

As it stands, the behavior is different from ERB without options, e.g.:

- no `-` without `trim_mode='-'`
- and no auto line removal without `trim_mode='<>'`

both of which erubis and Rails do by default.

So we should say that in the docs.