Captured vars don't work with indoc!, but work with printdoc!

According to the readme, `printdoc!($fmt, ...) — equivalent to print!(indoc!($fmt), ...)`, but this case is clearly different. Is there a way to have `indoc!` properly handle such cases for backward compatibility?

```rust
use indoc::{indoc, printdoc};

fn main() {
let s = "world";
println!(indoc!("Hello {}"), s);
printdoc!("Hello {s}");

println!(indoc!("Hello {s}"));
// error: there is no argument named `s`
// --> src/main.rs:8:21
// |
// 8 | println!(indoc!("Hello {s}"));
// | ^^^^^^^^^^^
// |
// = note: did you intend to capture a variable `s` from the surrounding scope?
// = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro

}
```

Reproduce the captured-variable examples in src/main.rs and compare the README's claimed equivalence between indoc! and printdoc!. Then trace how each macro handles the format string and determine whether backward-compatible support is possible; done means the reported case works or the limitation and rationale are documented.

Written by the indexing model from the issue text.