Iron out Rails/UniqBeforePluck

[pirj]

Follow-up to #580 (comment)

Removed detection of distinct after pluck:

> User.pluck(:email).distinct
(1.3ms)  SELECT "users"."email" FROM "users"
NoMethodError: undefined method `distinct' for ["[email protected]"]:Array

Improved examples and reasoning text.

Unfold specs.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• [-] Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• [-] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.
• [-] If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Thanks! 👍