There's no reason for guides to reference old behaviors. They should
be current as of the versions of Rails that they ship with, and including
older information just clutters thing.

I discussed this change with @fxn and he agrees.

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

Some guides work without 'the'. For instance, 'Migrations' or
'Form Helpers.' But when we talk about the asset pipeline, we...
always say 'the asset pipeline.' The guide title should reflect this.

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

This reverts commit d8596c4b.

Reason: too many mistakes [ci skip]

Fixes #2468.

This reverts commit f5b9ed4f.

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

This option is now unsupported in sprockets-rails.

In Textile `*` would convert to `<strong>`, but in Markdown we have to
use `**` instead.

In the text there is a section about how to serve the assets with
far-future headers. The snippet that is proposed doesn't work on Ubuntu
11 (probably others) as mod_expires is not enabled out-of-the-box. I
noted this, and gave an example of how to load mod_expires with a2enmod.

Selecting which key extensions to include in active_support/rails
made apparent the systematic usage of Object#in? in the code base.
After some discussion in

    https://github.com/rails/rails/commit/5ea6b0df9a36d033f21b52049426257a4637028d

we decided to remove it and use plain Ruby, which seems enough
for this particular idiom.

In this commit the refactor has been made case by case. Sometimes
include? is the natural alternative, others a simple || is the
way you actually spell the condition in your head, others a case
statement seems more appropriate. I have chosen the one I liked
the most in each case.