Changed the title of the 3.5 chapter from "Using the change Method" to
"When to Use the change Method" since the chapter tells the migration
definitions supported by the change method. This fix the duplicate ID in the
document.

I've also changed some of the titles to respect the Guides guidelines.

Add a paragraph about references statements in migration generator
and the subchapter "Supported type modifiers"

This reverts commit b11113f9.

Updated migrations.textile to change :integer on line 807 to :boolean, as it was earlier in the example. :flag is a boolean, not an integer. (defaults to false)

After 15fb4302 the default location of the :sql schema_format location was
changed from environment_name.sql to structure.sql. This pull request updates
a reference to the old schema format location in the migration guides.