Remove custom errors page section from the guides

This pattern is too problematic and introduces a lot of edge cases:

  * On 4.2, the issue https://github.com/rails/rails/issues/15124 is
    back again.
  * needs to define each action for each http status otherwise the
    router raises ActionController::RoutingError (No route matches).
  * If the router has `match "/*username",...` and some action is
    missing, Rails will pick up the "match" and try to do its job.
  * encourages people to copy & paste programming. Not DRY.

[ci skip]
fixing mailer previews for apps with globbing route
Add Guides note to change cache name space as Entry data is incompatible between 3.2 and 4.0
[ci skip] update rails version to 3.2.21
Mounted Rack apps should have default named routes based on app name

This fixes a regression in 4.2.0 from 4.1.8.

https://github.com/rails/rails/pull/17823 fixed a similar regression regarding _explicitly_ named routes for a mounted Rack app, but there was another regression for the default value.

With a route like:

    Rails.application.routes.draw do
      mount Mountable::Web, at: 'some_route'
    end

The "Prefix" column of rake routes gives the following:

- 4.1.8:         mountable_web
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     some_route   <- regression

This fixes the default to go back to being based off the name of the class like the docs specify: https://github.com/rails/rails/blob/785d04e3109f69d0b9b9f4732179592f0ef04e52/actionpack/lib/action_dispatch/routing/mapper.rb#L558-L560

Explicitly named routes still work correctly per https://github.com/rails/rails/pull/17823:

    Rails.application.routes.draw do
      mount Mountable::Web, at: 'some_route', as: 'named'
    end

- 4.1.8:         named
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     named
Merge pull request #17943 from jeremywadsack/doc_cache_importability

Add Guides note to change cache namespace
Merge pull request #17942 from yui-knk/fix/upgrading.md

[ci skip] update rails version to 3.2.21
Merge pull request #17944 from tjschuck/mounted_named_routes_regression

Mounted Rack apps should have default named routes based on app name
[ci skip] add `enable_extension` to PostgreSQL hstore example
Test coverage for ActiveJob generator
Merge pull request #17950 from y-yagi/postgres_guide

[ci skip] add `enable_extension` to PostgreSQL hstore example
Fix grammar of sentence in Reflection documentation
Merge pull request #17959 from eileencodes/fix-grammar-on-reflection

Fix grammar of sentence in Reflection documentation [ci skip]
minor sentences fixes
Make error message clearer that :on requires a symbol, not a string

The validation added in 5a3dc8092d19c816b0b1203945639cb91d065847 will
reject values for the `:on` option for after_commit and after_rollback
callbacks that are string values like `"create"`.

However, the error message says ":on conditions for after_commit and
after_rollback callbacks have to be one of create,destroy,update". That
looks like a string value *would* be valid.

This commit changes the error message to say ":on conditions for
after_commit and after_rollback callbacks have to be one of [:create,
:destroy, :update]", making it clearer that symbols are required.
Merge pull request #17963 from neerajdotname/wordsmitthing

minor sentences fixes
Add note to Store about uniqueness validation (#17954) [skip ci]
Merge pull request #17953 from kirs/test-activejob-generator

Test coverage for ActiveJob generator
Merge pull request #17966 from mikechau/update-ar-store-validation-documentation

Add note to Store about uniqueness validation (#17954) [skip ci]
Fix type case of "validations" and word-wrap from #17954 [ci skip]