doc/stdenv: Document autoPatchelfHook changes

First of all, this makes the existing documentation a bit more clear on
what autoPatchelfHook is all about, because after discussing with
@svanderburg - who wrote a similar implementation - the rationale about
autoPatchelfHook wasn't very clear in the documentation.

I also added the recent changes around being able to use autoPatchelf
manually and the new --no-recurse flag.

Signed-off-by: aszlig <aszlig@nix.build>