Add Haddock docs for new `class FiniteBits`
Add Haddock `/Since: 4.7.0.0/` comments to new symbols

These annotations were added in such a way, that the line

{{{
/Since: 4.7.0.0/
}}}

represents the last paragraph of the Haddock comment.

Maybe Haddock will have support for this meta-syntax at some point, and
be able to inherited the since-version property to the children of an
annotated symbol and display the since-version property in the rendered
documentation only in cases when it's not visually obvious (for
instance, when re-exporting documentation strings).

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>