[3.11] gh-93035: [Enum] Fix IntFlag crash when no single-bit members (GH-93076) (GH-93197)

`EnumType` attempts to create a custom docstring for each enum/flag, but that was failing with pathological flags that had no members (only multi-bit aliases).
(cherry picked from commit 08cfc3da)

Co-authored-by: Tobin Yehle <tobinyehle@gmail.com>