We should mention that the ordering matters with regards
to timestamps, and not imply that it's just for uniqueness
purposes.

Closes #8610.

This introduces a bunch of editing/re-writing to the migrations guide.
There were a bunch of small changes, and a few larger ones:

* mysql does support transactions.
* Add a section about db/seeds.rb
* Largely re-wrote the first few sections

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

Other guides that talk about Active Record specific features include the
name of the library in the title, this one should too for consistency.

Talked with @fxn about this. Bullet points should have periods at the ends.

The wording of these was a bit off, so I fixed them.

Most of the guides have a few simple sentences describing what they will show
you at the top. This one had a few big paragraphs. I've moved those paragraphs
down to an introductory one, and written a new smaller one for the introduction.
This makes this guide more consistent with the others.

Changed comment section to be more explicit. I had an issue with rolling back a migration and couldn't understand why 'rake db:reset' didn't work.

This reverts commit f5b9ed4f.

REASON: Translation work can not be done in the docrails repository

This is an attempt to address issue #6939, where an earlier migration
added a column to the database, and a later migration uses a model and
references that column.

When both migrations were run together with `rake db:migrate` the column
information in memory still referenced the old table structure.

Running the migrations separately fixed this, as a new connection was
then established before referencing the model. Explicitly calling
`reset_column_information` is a more reliable workaround.

The feature is available after Rails 3.1 and this is no longer a "new" feature.

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"