fetchGit: document shallow argument

src/libexpr/primops/fetchTree.cc

@@ -364,6 +364,10 @@ static RegisterPrimOp primop_fetchGit({
           A Boolean parameter that specifies whether submodules should be
           checked out. Defaults to `false`.
 
+        - shallow\
+          A Boolean parameter that specifies whether fetching a shallow clone
+          is allowed. Defaults to `false`.
+
         - allRefs\
           Whether to fetch all refs of the repository. With this argument being
           true, it's possible to load a `rev` from *any* `ref` (by default only