Update documentation for yml usage against https://github.com/rails/rails/issues/8612

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

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

The proper name, as per the documentation guides, is 'Active Record.' I've
updated several references to it as 'ActiveRecord'.

This reverts commit f5b9ed4f.

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

Change the default test locations to avoid confusion around the common
testing terms "unit" and "functional".
Add new rake tasks for the new locations, while maintaining backwards
compatibility with the old rake tasks.

New testing locations are as follows:

    app/models -> test/models (was test/units)
    app/helpers -> test/helpers (was test/units/helpers)
    app/controllers -> test/controllers (was test/functional)
    app/mailers -> test/mailers (was test/functional)

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

  # File lib/rack/response.rb, line 114
114:       def successful?;    @status >= 200 && @status < 300;       end

PATCH is the correct HTML verb to map to the #update action. The
semantics for PATCH allows for partial updates, whereas PUT requires a
complete replacement.

Changes:
* adds config.default_method_for_update you can set to :patch
* optionally use PATCH instead of PUT in resource routes and forms
* adds the #patch verb to routes to detect PATCH requests
* adds #patch? to Request
* changes documentation and comments to indicate support for PATCH

This change maintains complete backwards compatibility by keeping :put
as the default for config.default_method_for_update.