This removes the obsolete pre-Haddock2 `__HADDOCK__` conditional check
which isn't supported anymore by Haddock2 (and in fact the conditional
isn't needed anyway).

This commit also moves more content into the
`System/FilePath/Internal.hs` template that's `#include`d by
`System/FilePath/{Posix,Windows}.hs` and as a side-effect, a part of the
Haddock module-level documentation containing usage examples is now
shown again in the generated HTML output.

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