I had to do some I was unsure what kind of caching it was performing, and had to do some Googling. Also improves the wording.

`gmail` --> `Gmail`

[ci skip]

https://github.com/rails/rails-ujs is merged into actionview in favor of https://github.com/rails/rails/pull/28098.
[skip ci]

[ci skip]

Follow up to 79a5ea9e

Follow up to #24436

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.

Follow up to 625baa69

For more information about GNU zip mime type please check IETF's web site [RFC6713](http://tools.ietf.org/html/rfc6713) or [IANA](http://www.iana.org/assignments/media-types/media-types.xhtml#application)

Still more to do. Please assist!

`smpt.gmail.com` should be `smtp.gmail.com`

security measures so using the example for Gmail will return a “Password Incorrect” error,
and you will receive an email from Google that they blocked a sign-in attempt.  You can change
your Gmail settings or use another ESP.

I discovered this when I was testing a simple mailer example app and was just going to
use my personal Gmail account for the test.  I think it would be best to note this change
since now Gmail may not be the best option for a quick test.  I hope this saves time for other Rails
developers.  The Gmail example does show a good example of how to configure the smpt settings.

[ci skip]

[ci skip]

- Changed `IN` to `ON` in all note sentences in guides.

The single line was long enough that it rendered on two lines,
causing the example to look like syntactically invalid code.

When setting a mailer's default from address, you have to pass a hash
with a `:from` key; you can't pass just an email address.

References #18148.