Show source view and backtrace on missing template errors

This will help you debug missing template errors, especially if they
come from a programmatic template selection. Thanks to @dhh for
suggesting that.

As a bonus, also show request and response info on the routing error
page for consistency.

Mark comments that should not be in the docs [ci skip]

Some comments that are meant to separate blocks of code in a file show up
on http://api.rubyonrails.org as though they were part of the documentation.

This commit hides those comments from the documentation.

Stems from the discussion with @zzak at https://github.com/voloko/sdoc/issues/79#issuecomment-64158738

[ci skip]

[related #17718]

Thanks to @thedarkone for pointing out that an instance of this object
is used in a shared context.

Deprecate use route

Replace ActionDispatch::Head with Rack::Head.

Fix includes on association with a scope

Move test that depends on ActiveRecord

Fix wrong path in comments about stylesheets

I put the wrong path in my last PR by accident. Fixed here. Related to #17742

Change 'of plugins' syntax

Currently, the docs uses a syntax that is unclear and not general
American English. I've switched it to be clearer wording. Not a big
fix, but may be helpful.

[ci skip]

[ci skip] Fix class name of middleware.

Add a test for reflection keys as Strings, fixes #16928

See also PR: #17610

Specified beta branch for web-console gem

If the route set is empty, or if none of the routes matches with a score > 0,
there is no point showing the deprecation message because we are already be
raising the `ActionController::UrlGenerationError` mentioned in the warning.

In this case it is the expected behavior and the user wouldn't have to take any
actions.

Reference #17453

The internal tests that (incorrectly) relied on this were already fixed in
938d130c. However, we cannot simply fix this bug because the guides prior to
b7b9e920 recommended a workaround that relies on this buggy behavior.

Reference #17453

These are currently working "by accident" because `match_route` does not check
that the name is valid.

Reference #17453

Reference #17453

[Godfrey Chan, Washington Luiz]

Anchor should not be appended when set to nil/false.

[ci skip] Fix comment of ActionDispatch::Callbacks

cc ddce3dd3

cc #17719

Fix explanation for ActionDispatch::Callbacks. [ci skip]

ActionDispatch::Callbacks dose not run the prepare callbacks,
so change with comment on ActionDispatch::Callbacks.

Fixes 16928, improve reflections public API doc [ci skip]

`.reflections` public API changed to return a String instead of a Symbol
as keys.

see commit 1f314884 and 6259e4e2

[fixes #16928]
[fixes #17610]