Explain no expansion of ContextFunction0

[som-snytt]

Fixes #21321

Just an edge case.

I tried emitting TypeMismatch with addenda, but it didn't render, so that was too much hassle just to add a string.

This is the existing notice; it doesn't indicate why it doesn't work.

2 |val v1: scala.ContextFunction0[String] = "x" // error
  |                                         ^^^
  |                                         Found:    ("x" : String)
  |                                         Required: () ?=> String