Scaladoc: better support for context bounds in poly functions

[Florian3k]

Context bounds in poly functions, eg:

val less0: [X: Ord] => (X, X) => Boolean = ???

are currently documented as:

val less0: [X] => (X, X) => Ord[X] ?=> Boolean

They could be documented as in the source code:

val less0: [X: Ord] => (X, X) => Boolean

See #22019