message: Add more information about where repr might appear
diff: @@ -30,6 +30,20 @@ repr I32Be // normalizes to `Int`
  repr { x : U32Be } // normalizes to `{ x : Int } : Type`
  ```
  +The `repr` function will rarely appear in user-code,
  +but it may occasionally pop up in type errors when a `format` is unknown.
  +For example:
  +
  +```fathom
  +Things (format : Format): Format = {
  + thing : format,
  + // `thing : repr format` is now in the typing context
  + another_thing : Array (thing.len) U32Be, // error: `len` is not a field of `repr format`
  +};
  +```
  +
  +In the above example `format : Format` is unknown, and therefore its representation is also unknown.
  +
  ## Introduction
  ### Unsigned integer formats
  