This is technically not a sentence - there's no verb. Ditto for the two cells above. To me, it would be more consistent with e.g. the Hash and Pred entries to lowercase the two cells above this and remove the period at the end of all three.

This is also true for our "Returns:" elements, but we format those as sentences (with a leading capital letter and a trailing period).

It's fairly clear in that case that "Returns: Forty-two." is a stylized form of the sentence "Returns forty-two." Here, the words "[The] Return type [is]" needed to make this a sentence are much further away, and the majority of the cells don't include periods. The notion that the contents of the table cells are portions of larger sentences is a bit of a stretch.

That said, I don't find this usage particularly offensive. I'll let this one go so we can keep editorial focus on bigger issues.