Improve Rails console helper methods' descriptions

[st0012]

Motivation / Background

When demonstrating the result of #51705, I received some feedback around the description of the new helpers.

Detail

1. I made descriptions clearer around what components the methods are interacting with or return, which I think should make them easier to understand.
2. When updating the descriptions, I noticed that new_session has stopped taking block argument after Removed unwanted code which sets session host in console mode #44748, which nobody seems to discover and it's perhaps never used. And after that update, new_session is the same as app(true), which makes me think it has become obsolete and can be depracted.

Result

Helper methods
  conf           Returns the current IRB context.
  helper         Gets helper methods available to ApplicationController.
  controller     Gets a new instance of ApplicationController.
  new_session    [Deprecated] Please use `app(true)` instead.
  app            Creates a new ActionDispatch::Integration::Session and memoizes it. Use `app(true)` to create a new instance.

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[st0012]

I'm keeping 2 commits separated because one drops a helper.

[st0012]

@rafaelfranca I'd appreciate your input on this one 🙏

[st0012]

@rafaelfranca @carlosantoniodasilva 👋 would you consider having this in Rails 7.2 as well?