Warn if match in block is not used for PF

[som-snytt]

Fixes #21649

Addresses the immediate user issue. (Confusion about compiler behavior.)

Follow-up is improve doc for PartialFunction and improve spec for pattern-matching anonymous function to include non-trivial selector expression.

Actually, first follow-up will be to address:

scala> def f: PartialFunction[(Int, Int), Int] = (x, y) => (2*x, y) match { case (x, y) if x < 5 => x + y }
1 warning found
-- [E210] Syntax Warning: ------------------------------------------------------
1 |def f: PartialFunction[(Int, Int), Int] = (x, y) => (2*x, y) match { case (x, y) if x < 5 => x + y }
  |                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |match expression in result of block will not be used to synthesize partial function
  |
  | longer explanation available when compiling with `-explain`
def f: PartialFunction[(Int, Int), Int]

oh that's because of how parameters are untupled, no doubt. Compare

scala> def f: PartialFunction[(Int, Int), Int] = p => (2*p._1, p._2) match { case (x, y) if x < 5 => x + y }
def f: PartialFunction[(Int, Int), Int]

[som-snytt]

footnote the syntax was aligned intentionally 8a91774 which is where I picked up the phrase non-trivial selector expression.