`:doc` should produce help text for `each` and `filter`

[byorgey]

Currently e.g. :doc each only shows its type:

Disco> :doc each
This expression has multiple possible types.  Some examples:
each : (a1 → a) × List(a1) → List(a)
each : (a1 → a) × Bag(a1) → Bag(a)
each : (a1 → a) × Set(a1) → Set(a)