Improve `StrEnum` documentation

[nachocab]

The StrEnum docs include a note that references str(StrEnum.member), which can seem like member is a method of the StrEnum class.

Also, unlike IntEnum, there is no illustrative example.

Linked PRs

• gh-136859: Improve StrEnum docs #136864
• [3.14] gh-136859: Improve StrEnum docs (GH-136864) #136936
• [3.13] gh-136859: Improve StrEnum docs (GH-136864) #136937