Add documentation for RSpec/ExampleLength configuration option CountAsOne: [method_call]

[ydah]

Fix: #1728

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• [-] Added tests.
• Updated documentation.
• [-] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[Darhazer]

The option method_call appeared in Rubocop 1.40. Should we also update rubocop-rspec to require minimum 1.40 (currently 1.33) for rubocop? Otherwise documentation will be potentially misleading

[ydah]

@Darhazer Indeed it is, I updated this PR. Thank you!

[pirj]

Thank you!