Document Token{Stream,Tree}::Display more thoroughly.

To expressly warn against the kind of proc macro implementation that was
broken in rust-lang#119875.

library/proc_macro/src/lib.rs

@@ -191,6 +191,14 @@ impl ToString for TokenStream {
 /// Prints the token stream as a string that is supposed to be losslessly convertible back
 /// into the same token stream (modulo spans), except for possibly `TokenTree::Group`s
 /// with `Delimiter::None` delimiters and negative numeric literals.
+///
+/// Note: the exact form of the output is subject to change, e.g. there might
+/// be changes in the whitespace used between tokens. Therefore, you should
+/// *not* do any kind of simple substring matching on the output string (as
+/// produced by `to_string`) to implement a proc macro, because that matching
+/// might stop working if such changes happen. Instead, you should work at the
+/// `TokenTree` level, e.g. matching against `TokenTree::Ident`,
+/// `TokenTree::Punct`, or `TokenTree::Literal`.
 #[stable(feature = "proc_macro_lib", since = "1.15.0")]
 impl fmt::Display for TokenStream {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -758,6 +766,14 @@ impl ToString for TokenTree {
 /// Prints the token tree as a string that is supposed to be losslessly convertible back
 /// into the same token tree (modulo spans), except for possibly `TokenTree::Group`s
 /// with `Delimiter::None` delimiters and negative numeric literals.
+///
+/// Note: the exact form of the output is subject to change, e.g. there might
+/// be changes in the whitespace used between tokens. Therefore, you should
+/// *not* do any kind of simple substring matching on the output string (as
+/// produced by `to_string`) to implement a proc macro, because that matching
+/// might stop working if such changes happen. Instead, you should work at the
+/// `TokenTree` level, e.g. matching against `TokenTree::Ident`,
+/// `TokenTree::Punct`, or `TokenTree::Literal`.
 #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
 impl fmt::Display for TokenTree {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {