pkgsLLVM.git-doc: improve eval error (#474199)

pkgs/top-level/all-packages.nix

@@ -1156,13 +1156,18 @@ with pkgs;
   # Git with SVN support, but without GUI.
   gitSVN = lowPrio (git.override { svnSupport = true; });
 
-  git-doc = lib.addMetaAttrs {
-    description = "Additional documentation for Git";
-    longDescription = ''
-      This package contains additional documentation (HTML and text files) that
-      is referenced in the man pages of Git.
-    '';
-  } gitFull.doc;
+  git-doc =
+    # doc attribute is not present at least for pkgsLLVM
+    if (gitFull ? doc) then
+      lib.addMetaAttrs {
+        description = "Additional documentation for Git";
+        longDescription = ''
+          This package contains additional documentation (HTML and text files) that
+          is referenced in the man pages of Git.
+        '';
+      } gitFull.doc
+    else
+      throw "'git-doc' can't be evaluated as 'gitFull' does not expose 'doc' attribute";
 
   gitMinimal = git.override {
     withManual = false;