Fix attaching docstrings to where call syntax (#33446)
* Allow documenting where-expressions
I.e. docstrings can now also be attached to expressions that look as
follows:
"""
"""
foo(::T) where T
Note that it was already fine to attach docstrings to such methods when
the docstring was attached to the function body:
"""
"""
function foo(::T) where T
...
end
* Better fix
* Add tests
