`lib.customization.extendMkDerivation` not found

**What happened** I went searching for `lib.customization.extendMkDerivation`, added in https://github.com/NixOS/nixpkgs/pull/234651 about a month ago. I didn't find it! **Steps to Reproduce** 1. Search for `extendMkDerivation` **Expected behavior** The documentation (see also https://nixos.org/manual/nixpkgs/unstable/#function-library-lib.customisation.extendMkDerivation) I also expected to find a reference to the commit of nixpkgs that Noogle was using, and didn't find that either.