doc: listToAttrs: document repeated keys

[aakropotkin]

Previously I had thought this was implemented as the first example below ( I was wrong ):

# WRONG
listToAttrs = attrs:
  builtins.foldl' ( acc: key: acc // { ${key} = attrs.${key}; } ) {}
                  ( builtins.attrNames attrs );
# RIGHT
listToAttrs = attrs:
  builtins.foldl' ( acc: key: { ${key} = attrs.${key}; } // acc ) {}
                  ( builtins.attrNames attrs );

I was browsing the primops.cc sources and bumped into an inline comment that mentioned that it was actually the opposite behavior: listToAttrs preserves the "first instance" of a key.

This change adds a note to the docstring, and extends the existing example to make the behavior clear to users.

[fricklerhandwerk]

Great catch, thank you!

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2022-12-02-nix-team-meeting-minutes-13/23731/1