Add context to output of GHCi :doc command

Currently the GHCi :doc command simply prints the haddocks for all objects matching the argument string. This means that there is no indication of which object a given block of text belongs to when multiple results are printed.

As was briefly discussed in !4562 (closed), it would be useful to include some additional output showing what the object is and where it is defined. For example,

-- | haddock for type con
newtype Foo =
  -- | haddock for data con
  Foo
    { bar :: Int -- ^ haddock for field
    }

>>> :doc Foo
-- defined at Docs.hs:7:3
pattern Foo :: Int -> Foo
  haddock for data con

-- defined at Docs.hs:5:1
data Foo :: Type
  haddock for type con

Note the additional output clearly identifying the subject of each doc.

added needs triage label

added GHCi Pnormal Tbug labels and removed needs triage label

I'll be working this issue. Please let me know if there are any further suggestions for the :doc output.

mentioned in merge request !4688 (closed)

mentioned in commit b4880d40

mentioned in commit 3c822b6a

mentioned in commit babcaf34

mentioned in commit 93ae0e2a

Done in !4688 (closed).

closed