Workaround for the OMERO session-dir bug.

fixes #294
Minor re-phrasing, typo and layout fixes.
Adapt to supported distributions only to reduce confusion.
Fix mixed config/explanation syntax bugs.

Explanations were indented like code blocks and therefore rendered as part
of the config files instead of separate text.
Close PHP blocks to avoid reST syntax problems.

Plus some minor cleanups.
Fix typo in AD-forests/multidomain section.