Explain read-only consumes

 - New section in separation checking doc page
 - Slight generalization of expectsReadOnly
 - Lots of tests variations of #24373

Author: odersky

tests/neg/i24375.scala

@@ -0,0 +1,2 @@
+def foo(f: (x: Int) ?=> Int) = f(using 0)
+@main def Main = println(foo(x))