It's useful to have a link to the release notes in the upgrade guide for
a clearer view of what changed in Rails.

Make release notes about legacy `mysql` adapter clearer.

Adds missing upgrade items to the upgrade guides for Rails 5. Fixes some
typos and whitespace as well.

Follow up to 5f777e4b

These comments were removed in 8b69f1ee.

Fix a tiny typo and vertical-align some return results in the
ActiveModel::Errors documentation.

[ci skip]

r? @rafaelfranca

This will make easier to applications that rely on having const_missing
hooks in production upgrade to Rails 5.

This option is going to be remove in the future and the default behavior
will be to disable the dependency loading.

- Fixes #25394.

reverts change in previous commit
https://github.com/rails/rails/commit/ea4f0e2

docs should prefer to use `rails` over `rake` in the context of an
application, but in the context of the Rails source this is not the case

The beginning of the note on the :on option is not capitalised correctly.
The :destroy symbol in the after_commit example is unnecessarily wrapped in an array.

@prathamesh-sonpatki

- https://github.com/rails/rails/pull/18288.
- https://github.com/rails/rails/commit/da9038e.
- https://github.com/rails/rails/commit/78dab2a8569408658542e462a957ea5a35aa4679.
- https://github.com/rails/rails/pull/9065.
- https://github.com/rails/rails/pull/18314.
- https://github.com/rails/rails/pull/13434.

- Followup of https://github.com/rails/rails/commit/8b69f1eeba753c38364fb88136b2503480f2de1d.

Link to the more detailed message documentation when using a message option in validations.

- It is also marked as WIP as we are still refining it.

- https://github.com/rails/rails/pull/18721
- https://github.com/rails/rails/pull/11898
- https://github.com/rails/rails/pull/18194
- https://github.com/rails/rails/pull/18225
- https://github.com/rails/rails/pull/18244
- https://github.com/rails/rails/commit/edc2b7718725016e988089b5fb6d6fb9d6e16882
- https://github.com/rails/rails/pull/19686

SSL_set_verify(3) explains:

SSL_VERIFY_FAIL_IF_NO_PEER_CERT
  Server mode: if the client did not return a certificate, the TLS/SSL
handshake is immediately terminated with a "handshake failure" alert.
This flag must
  be used together with SSL_VERIFY_PEER.

  Client mode: ignored

SSL_VERIFY_CLIENT_ONCE
  Server mode: only request a client certificate on the initial TLS/SSL
handshake. Do not ask for a client certificate again in case of a
renegotiation.
  This flag must be used together with SSL_VERIFY_PEER.

  Client mode: ignored

The SMTP connection here uses a OpenSSL socket in client mode,
suggesting invalid/ignored flags is rather misleading.

[ci skip]

it reads a bit awkwardly without it.

Not sure why #25239, but it had good changes, so I'm manually pulling in
the changes.

[Jon Moss, Rajat Bansal]

[ci skip]

[ci skip]

`:enum_prefix` and `:enum_suffix` was changed to `:_prefix` and `:_suffix`
in #20999.

[ci skip]