Describe the interaction between input addressing and content addressing that leads to 1:many relation between output and derivations

[roberth]

Problem

• Store path metadata has the potential to get corrupted #13384 was hard to explain.

Proposal

Elaborate on my explanation for the behavior in #13384 in the manual.

1. think about how this relates to floating ca outputs
   • https://nix.dev/manual/nix/2.29/store/derivation/outputs/content-address.html#design-rationale almost leads us there, but doesn't go into the consequences for input addressed derivations (probably not the right place for the new content, but a "see also"-like link seems appropriate).
2. depending on (1) integrate the explanation it into the currently very short page about input addressing, or somewhere else
   • omit the "fresh derivation" and "override" parts, which are not relevant to the hashing machinery
3. clarify with a concrete example

Checklist

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

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