Rollup merge of #95979 - lcnr:coherence-docs, r=compiler-errors

update coherence docs, fix generator + opaque type ICE

the world is confusing, this makes it slightly less so

compiler/rustc_middle/src/traits/mod.rs

@@ -61,6 +61,9 @@ pub enum Reveal {
     ///     let x: <() as Assoc>::Output = true;
     /// }
     /// ```
+    ///
+    /// We also do not reveal the hidden type of opaque types during
+    /// type-checking.
     UserFacing,
 
     /// At codegen time, all monomorphic projections will succeed.