Add Haddock `/Since: 4.6.0.0/` comments to symbols

This commit retroactively adds `/Since: 4.6.0.0/` annotations to symbols
newly added/exposed in `base-4.6.0.0` (as shipped with GHC 7.6.1).

See also 6368362f which adds the respective annotation for symbols newly
added in `base-4.7.0.0` (that goes together with GHC 7.8.1).

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