EDAC: Update Documentation/edac.txt

Do some initial cleanup, more probably will come.

- Move credits section to the end
- Update maintainers
- Drop sourceforge reference - project is long upstream now
- Reformat sections
- Reformat paragraphs
- Clarify text
- Bring it up-to-date
- Drop useless "future hardware scanning" section
Signed-off-by: Borislav Petkov <bp@suse.de>