Improve the "input and output are the same file" error text (#12619)

# Description
Continuing from #12601, this PR improves the error message help text and
adds an example to `collect`.

crates/nu-command/src/filesystem/save.rs

@@ -260,7 +260,7 @@ fn saving_to_source_file_error(dest: &Spanned<PathBuf>) -> ShellError {
             dest.item.display()
         ),
         span: Some(dest.span),
-        help: Some("you should change the output path".into()),
+        help: Some("You should use `collect` to run your save command (see `help collect`). Or, you can put the file data in a variable and then pass the variable to `save`.".into()),
         inner: vec![],
     }
 }