Documentation: `strictDeps`

[l0b0]

Problem

There's nothing about strictDeps in the nixpkgs manual, despite it being used in seemingly hundreds of places.

Proposal

Document what this flag does, and when to use it.

Checklist

• checked latest Nixpkgs manual (source) and latest NixOS manual (source)
• checked open documentation issues for possible duplicates
• checked open documentation pull requests for possible solutions

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