docs(feat/derivation): add docstring to formatter

This is just to clarify that the formatted result is the ATerm representation

Change-Id: I98fd0b1d2daa3cf6fdbe526ae1e0bd100ff62df3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7742
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/derivation/src/derivation.rs

@@ -34,6 +34,7 @@ impl Derivation {
 }
 
 impl fmt::Display for Derivation {
+    /// Formats the Derivation in ATerm representation.
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.serialize(f)
     }