fetchTree: make invisible when not available, sorta document

Documenting fetchTree is an exercise in frustration because of the sheer
amount of stringly typed everything in it. I do not know which fields
exist without reading the entirety of libfetchers. However, we can write
something slightly perfunctory but at least perhaps somewhat helpful
documentation-wise.

Fixes: https://git.lix.systems/lix-project/lix/issues/609
Change-Id: I991391b53fcd69172dbc7efb9d384e62bc847b91

lix/libexpr/primops/fetchTree.cc

@@ -199,19 +199,11 @@ static void fetchTree(
     emitTreeAttrs(state.ctx, tree, input2, v, params.emptyRevFallback, false);
 }
 
-static void prim_fetchTree(EvalState & state, const PosIdx pos, Value * * args, Value & v)
+void prim_fetchTree(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    experimentalFeatureSettings.require(Xp::Flakes);
     fetchTree(state, pos, args, v, std::nullopt, FetchTreeParams { .allowNameArgument = false });
 }
 
-// FIXME: document
-static RegisterPrimOp primop_fetchTree({
-    .name = "fetchTree",
-    .arity = 1,
-    .fun = prim_fetchTree
-});
-
 static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v,
     const std::string & who, bool unpack, std::string name)
 {