Missing documentation: `unsafeDiscardOutputDependency`

[roberth]

Problem

It's only been documented as

• being the inverse of addDrvOutputDependencies
• "derivation deep"

Proposal

Basically say that it causes the outputs to be built.

Checklist

• [x] checked latest Nix manual (source)
• [x] checked open documentation issues and pull requests for possible duplicates

Priorities

Add :+1: to issues you find important.