[Follow-up] Fix tags and details rubocop linters

The following discussion from !209136 (merged) should be addressed:

• @mwoolf started a discussion: (+2 comments)

  issue: This one isn't working as expected 🤔 Might need debugging before we can merge.

  In general though, this looks good @vwolanyk!

  

Summary

• when only 1 element exists in a desc block the AST that rubocop parses to match node patterns is structured differently than if there were multiple items/method calls in the block
• this means that if a desc block only has one element it can return a false negative and fail the linter for that rule

Tasks

• Update Rubocop::Cop::API::DescriptionDetail to handle this edgecase
• Update Rubocop::Cop::API::DescriptionTags to handle this edgecase