With the upcoming 2.1 [web-console release], I have tweaked the existing
documentation a bit. I tried to focus on the spawning console use case,
because I think a lot of people overlook that.

Tried to explain it as best as I can, however, my English and prose
aren't my best skills :) If you guys can chime in and help me improve
the wording, I would be extremely grateful.

There has been some configuration default changes. For example, the
whitelisted IPs always include IPv4 and IPv6 localhosts now and this
wasn't the case in 2.0. I think a lot of people got bitten by it, that's
why I changed it. I'm a bit confused on how to document this. Should I
just document the latest version, I don't think I can expect all the
people reading the guide to be on it.

[web-console release]: https://github.com/rails/web-console/pull/110

improve sentence flow with grammar change

document web console in debugging guide [ci skip]

document web console in debugging guide [ci skip]

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

References #18148.

As discussed in #15304, we need to automate this process but for now,
having out-of-date versions is not ideal.

Since master targets 4.2.0, let's also update references to the last 4.1
version to 4.2.0.

Finally, let's remove mentions to versions when this is not needed. The
guides cover the features of the current version anyway.

[Juanito Fatas + Robin Dupret]

This is a follow up to 545afc15.

Tested on a brand new app to exactly match current byebug's behaviour.

[ci skip]

* Indent after private/protected
* Ruby >= 1.9 syntax for hashes
* Prefer method { do_stuff } instead of method{do_stuff} for single-line blocks.

[ci skip]

* it's Action Mailer not ActionMailer
* it's Action Pack not ActionPack
* it's Active Support not ActiveSupport

/cc @fxn

see: https://github.com/blog/738-sidejack-prevention-phase-2-ssl-everywhere

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

Talked with @fxn about this. Bullet points should have periods at the ends.