Improvements to Common Array Methods Ruby guide (#24705)
Improvements to Common Array Methods Ruby guide (#24705)

- Edits to the intro paragraph for shorter and clearer sentences.
- Updated the `.length` description to refer to it as *the* array rather than *your*, to be consistent with other descriptions.
- For `.first` and `.last`, I updated the wording from "accesses" to "returns".
- Added clarification that `.pop` returns the removed element (since `.shift` also mentions this).
- Updated the code snippet for `.select`, which mistakenly implied that the original array is modified
- Added clarification that `.include?` returns `true` if the array includes the element.
- The section for `.flatten` was stuck in the code snippet for `include?`, I've fixed that.
- Added clarification that `.uniq` does not modify the original array.
- Updated the `.uniq` description by cutting extra wording. `.uniq` doesn't take an array, it's called on an array.
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