Add example to docs/language/informal/super-bounded-types.md.
Add example to docs/language/informal/super-bounded-types.md.

This CL adds an example to super-bounded-types.md in order to motivate
the definition of what it means for a parameterized type based on a
type alias to be super-bounded. The definition says that we must check
the actual type arguments relative to the formal type parameter
list of the given `typedef` as well as the right hand side, replacing
the formals by the given actual type arguments. The example shows that
the right hand side can be ill-bounded even though the check that we
apply based on the formal type parameter bounds (that is, the check
that we apply to class types) succeeds.

A rendered version of super-bounded-types.md corresponding to patchset
6 is available here:
https://gist.github.com/eernstg/fc12eeb23064a2578a936b443461dde4.

Change-Id: I33dc6ced592f53160bc6f933558bfface46cd329
Reviewed-on: https://dart-review.googlesource.com/56668
Commit-Queue: Erik Ernst <251bb01623bfda2cd9151e339a6e783ca432874c@google.com>
Reviewed-by: Leaf Petersen <9bef692239b5f624cec8e2cd5249ab1939a377aa@google.com>

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