Unstable "use" does not interoperate well with documentation of underlying types

[sandersaares]

When a re-export "use" statement is marked as unstable, the availability chapter seems to get just prepended to the original docs without any proper formatting to separate it from the original body. This seems suboptimal from a readability perspective - I would expect it not to "take over" the documentation like that:

I am guessing this is just how doc attributes on re-exports are handled - prefix the original with it? But at least some improvements seem possible here:

• Do not start with an "Availability" chapter - the original text in there is not about availability.
• Start a new paragraph after the availability warning, so there is a clear distinction between availability warning and type documentation.