Update script type (default) and further clarify

The documentation assumes a _.template style pre-compilation, while I was only familiar with pre-compilers that parsed script with a type of `text/template`. The documentation should reflect that more clearly, given the presence of the latter style.

This commit removes the incorrect type of `text/template` and adds another line to the example's explanation to make it more clear the the pre-compiler will be expanding the contents of the script passed in:

```
For example, using a pre-compiler with ERB-style delimiters,
the source can be made to populate a script tag:
```