[ci skip]

<plus> is nonstandard Textile; RailsGuides::TextileExtensions::plusplus
is the method that causes multiple code-formatted `+` symbols to
generate crap like

    <code>1 <notextile><tt>+</tt></notextile> 2 ...</code>

instead of the expected output:

    <code>1 + 2 ...</code>

[getting started] some more handholding around where exactly the error would appear when a post is invalid

[getting started] re-add new action back to PostsController, explain that it'll be explained in a short while

This reverts commit 3b6c0e35.

Reason: changelogs aren't meant to be edited in docrails.

Fix an error in methods example of instanciated fixtures.

Conflicts:
	actionpack/lib/action_view/helpers/asset_tag_helper.rb