Annex on specific OS should be removed

[Jeff Johnson]

Annex on specific OS should be removed and their content spread across the FHS
document to allow for a reader to find all information on a given directory in
a single place.

It is also removing analysis error when the reader would not know that such an
annex exist and will only refer to the paragraph of the directory concerned,
thus missing specific points.

These notes are often short, so that won't clobber the paragraph hosting them.

Typically, as an example 6.1.2. content should go under 3.4