Empty Haddock comments no longer occur in the AST as `HsDoc`
## Summary
Consider the following two type signatures.
```haskell
foo :: {- |-} A -> B
bar :: {- | -} A -> B
```
Comparing the AST (with `-haddock`) of `foo` and `bar`, note that `foo` does not contain a `HsDoc` (searchf or `WithHsDocIdentifiers`), but `bar` does:
<table>
<tr><th>
`foo`</th><th>
`bar`</th></tr>
<tr>
<td>
```haskell
(L
(SrcSpanAnn (EpAnn
(Anchor
{ <interactive>:1:1-20 }
(UnchangedAnchor))
(AnnListItem
[])
(EpaComments
[])) { <interactive>:1:1-20 })
(SigD
(NoExtField)
(TypeSig
(EpAnn
(Anchor
{ <interactive>:1:1-3 }
(UnchangedAnchor))
(AnnSig
(AddEpAnn AnnDcolon (EpaSpan { <interactive>:1:5-6 }))
[])
(EpaComments
[]))
[(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:1-3 })
(Unqual
{OccName: foo}))]
(HsWC
(NoExtField)
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:15-20 })
(HsSig
(NoExtField)
(HsOuterImplicit
(NoExtField))
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:15-20 })
(HsFunTy
(EpAnn
(Anchor
{ <interactive>:1:15 }
(UnchangedAnchor))
(NoEpAnns)
(EpaComments
[]))
(HsUnrestrictedArrow
(L
(TokenLoc
(EpaSpan { <interactive>:1:17-18 }))
(HsNormalTok)))
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:15 })
(HsTyVar
(EpAnn
(Anchor
{ <interactive>:1:15 }
(UnchangedAnchor))
[]
(EpaComments
[]))
(NotPromoted)
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:15 })
(Unqual
{OccName: A}))))
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:20 })
(HsTyVar
(EpAnn
(Anchor
{ <interactive>:1:20 }
(UnchangedAnchor))
[]
(EpaComments
[]))
(NotPromoted)
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:20 })
(Unqual
{OccName: B}))))))))))))
```
</td>
<td>
```haskell
(L
(SrcSpanAnn (EpAnn
(Anchor
{ <interactive>:1:1-21 }
(UnchangedAnchor))
(AnnListItem
[])
(EpaComments
[])) { <interactive>:1:1-21 })
(SigD
(NoExtField)
(TypeSig
(EpAnn
(Anchor
{ <interactive>:1:1-3 }
(UnchangedAnchor))
(AnnSig
(AddEpAnn AnnDcolon (EpaSpan { <interactive>:1:5-6 }))
[])
(EpaComments
[]))
[(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:1-3 })
(Unqual
{OccName: bar}))]
(HsWC
(NoExtField)
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16-21 })
(HsSig
(NoExtField)
(HsOuterImplicit
(NoExtField))
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16-21 })
(HsFunTy
(EpAnn
(Anchor
{ <interactive>:1:16 }
(UnchangedAnchor))
(NoEpAnns)
(EpaComments
[]))
(HsUnrestrictedArrow
(L
(TokenLoc
(EpaSpan { <interactive>:1:18-19 }))
(HsNormalTok)))
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16 })
(HsDocTy
(EpAnnNotUsed)
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16 })
(HsTyVar
(EpAnn
(Anchor
{ <interactive>:1:16 }
(UnchangedAnchor))
[]
(EpaComments
[]))
(NotPromoted)
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:16 })
(Unqual
{OccName: A}))))
(L
{ <interactive>:1:8-14 }
(WithHsDocIdentifiers
(NestedDocString
(HsDocStringNext)
(L
{ <interactive>:1:8-14 }
(HsDocStringChunk
" ")))
[]))))
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:21 })
(HsTyVar
(EpAnn
(Anchor
{ <interactive>:1:21 }
(UnchangedAnchor))
[]
(EpaComments
[]))
(NotPromoted)
(L
(SrcSpanAnn (EpAnnNotUsed) { <interactive>:1:21 })
(Unqual
{OccName: B}))))))))))))
```
</td>
</tr>
</table>
Is there a particular reason for this? In GHC 8.10, the AST contained Haddock comments in both cases.
Concrete effects of this behavior:
- It makes the job of formatters like Ormolu (see issues [1068](https://github.com/tweag/ormolu/pull/1068), [1065](https://github.com/tweag/ormolu/issues/1065), [726](https://github.com/tweag/ormolu/issues/726)) that check of AST discrepancies automatically harder than necessary, as eg a natural rewrite from
```haskell
foo ::
-- |
--
A ->
B
```
to
```haskell
foo ::
-- |
A ->
B
```
contains a Haddock comment in the AST in the first snippet, but not in the second.
- A nice Haddock trick by @tomjaguarpaw1 ([blog post](http://h2.jaguarpaw.co.uk/posts/improving-the-typed-process-documentation/), search for "Forced type signatures to wrap") does [no longer work](https://github.com/tweag/ormolu/pull/1068#issuecomment-1707237587).
Ideally, the behavior would be changed as it was in 8.10; I could try to do that in case this behavior is not intentional.
## Environment
* GHC version used: Any GHC since 9.0 (I think this change is due to !2377)
issue