ELIXIR FORMATTER DOCS FINALLY CATCH UP TO v1.15.0

Sigil callbacks have had access to an :opening_delimiter option in their opts for over a year [1], but it was never listed in the official Mix.Tasks.Format documentation alongside :sigil and :modifiers. This option lets your formatter distinguish between different sigil forms, like a heredoc sigil versus a regular one. If you're building custom formatters or need fine-grained control over how sigils are rendered, this changes what's possible. The documentation gap meant many developers either rediscovered this by digging through source code or shipped formatters without leveraging it. This is the kind of quiet capability that suddenly makes sense once you know it exists.