Document @:callable

I was looking at how allow an abstract of function to be called, this is implemented with `@:callable` but does not seem to be documented atm. Maybe @Simn you could review the newest metadata that were recently added for abstracts?

Start by locating the manual section covering abstract metadata and review how related, recently added metadata are documented. Document the @:callable metadata and explain how it allows an abstract function to be called; done means the feature is covered in the appropriate manual section.

Written by the indexing model from the issue text.