Error messaging for non `Lit::str` doc comments

[20jasper]

When anything other than a Lit::str is passed to a doc attribute, displaydoc will panic with "not implemented" instead of instructing the user to pass a literal

There are some reasonable usecases for non literals like include_str! to be used in doc attributes. It seems in a prior version, there was a more specific message, as seen in #36

What do y'all think of providing a more specific message instead of using a raw unimplemented! macro? Alternatively, a select list of macros like concat!, include_str! could be evaluated further, though of course this option is far more complex

Minimal reproduction

#[derive(Display)]
#[doc = include_str!("./hi.txt")]
struct Foo;

error: proc-macro derive panicked
 --> tests/ui/non_string_doc_expr.rs:3:10
  |
3 | #[derive(Display)]
  |          ^^^^^^^
  |
  = help: message: not implemented