docs(tvix/eval): update string context document r/8912

NixString doesn't contain a `HashSet<BuildReference>` anymore, there's a
more packed internal data structure. We don't need to be overly specific
in the docs, just say we expose an API.

Change-Id: I13380c49293f9a86d1916909fdfeefbe64d9024b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12755
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: edef <edef@edef.eu>

0 files changed, 0 insertions, 0 deletions