Tweak document and message for Style/FlipFlop cop

[koic]

flip-flop operator is deprecated since Ruby 2.6.0.

The flip-flop syntax is deprecated. [Feature #5400]

• https://github.com/ruby/ruby/blob/v2_6_0/NEWS#language-changes
• https://bugs.ruby-lang.org/issues/5400

% cat example.rb
(1..20).each do |x|
  puts x if (x == 5) .. (x == 10)
end

% ruby -v
ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-darwin17]

% ruby example.rb
example.rb:2: warning: flip-flop is deprecated
5
6
7
8
9
10

This PR tweaks document and offense message for Style/FlipFlop cop.
It will more clarify the reason for checking flip-flop operator.

---

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 to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• 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.

[bbatsov]

I guess now that's a lint cop then.

[bbatsov]

Thanks!

[koic]

I guess now that's a lint cop then.

Sure! I will move this cop from Style department to Lint department.