garbage-collection internals are not documented / toString produces store path without context

[DaniD3v]

garbage-collection internals are not documented / toString produces store path without context

[roberth]

Technically this is about the expression language semantics more so than the store-level garbage collection behavior.
I honestly don't know how we should address this in the manual. Where would you expect to find this information?

As for the behavior, toString traditionally did not need to return a string with a string context, because e.g. toString /home == "/home".
It seems likely that you're using Flakes, which makes this behavior easy to encounter by putting sources in the store first.
Based on the discussion in #8428, I think the best way to prevent this mistake is not to create path values (what you referred to by relative paths, iiuc) that refer to the store.

Regardless, the toString documentation is lacking.