Normally I would have done this in master, but there was already
a guide with wrapped content, so worked here. I am going to
cross-merge now.

This reverts commit e3dbe66e.

Reason: Code changes are not allowed in docrails.

- includes 3 tests

Reason: <notextile> is an internal tag

This reverts commit 01040b5d.

This reverts commit 3e6052a4.

Reason: Code changes are not allowed in docrails (even if they are related to guides generation)

After a long list of discussion about the performance problem from using varargs and the reason that we can't find a great pair for it, it would be best to remove support for it for now.

It will come back if we can find a good pair for it. For now, Bon Voyage, `#among?`.

There're a lot of places in Rails source code which make a lot of sense to switching to Object#in? or Object#either? instead of using [].include?.

If you're a guide writer and want to generate the guides, Run :

  ruby railties/guides/rails_guides.rb

And guides HTML will get generated inside railties/guides/output directory.