DOC: major reorg of ACL + sample fetch

DOC: major reorg of ACL + sample fetch

The split between ACL and sample fetch was a terrible mess in the doc,
as it caused all entries to be duplicated with most of them not easy to
find, some missing and some wrong.

The new approach consists in describing the sample fetch methods and
indicating the ACLs that are derived from these fetches. The doc is
much smaller (1500 lines added, 2200 removed, net gain = 700 lines)
and much clearer.

The description of the ACL mechanics was revamped to take account of
the latest evolutions and clearly describe the compatibility between
types of fetches and ACL patterns.

The deprecated keywords have been marked as such, though they still
appear in the examples given for various other keywords.