Explain ERB space removal.

The current phrase about space removal is misleading, and too hard to understand without an example. I propose that we either:

- explain it better
- remove it completely and let people look up erubis docs now that we linked to them at https://github.com/rails/rails/pull/16774

It is misleading because:

- spaces are only removed if the entire line contains only whitespaces
- `-%>` is not interchangeable with `%>` when using the equals sign `<%= %>`.

If you think the new explanation is too long, we can consider removing everything after "The line is not removed when" as that use case is less common: at least the new information won't be ambiguous and will have a clarifying example.