more fixups for GHC docs: ddump flags

[Rene de Visser]

The flags under section

4.17.26. Compiler debugging options

seem to be out of date. They seem to be wrong for both 6.42 and 6.6

For example --ddump-cmm is not listed, but works.

But --ddump-absC is listed, and gives the error unknown flag.

I can't get -ddump-stix to work either.

Gone, or renamed, or only works in combination with other flags?