DOC: filters: document the automatic ordering of the legacy core filters

DOC: filters: document the automatic ordering of the legacy core filters

The default evaluation orders of the filters are now documented in the
configuration manual, next to the "filter-config" documentation, for both
filter modes: the default placement of the built-in classes in the new
mode, and the automatic placement of the implicitly declared core filters
in the legacy mode, as introduced by the automatic ordering. For each
mode, the three orders are given: the global one, used for the
side-independent operations, and the request and response ones, used for
the evaluation order of the filters on the corresponding channel.

The "decompression" directives no longer recommend declaring the
decompression filters explicitly to prevent misconfigurations: when
implicitly enabled in the legacy filter mode, the decompression filter is
automatically placed at the beginning of the filter list, before the
cache and compression filters.

Both modes' orders are explicitly informational only: they describe the
current placement and they may be changed by the build options or by a
future version. Only the placements explicitly requested by a filter, and
the position of an implicitly declared legacy filter relative to the
explicitly declared ones, should be relied upon.

The filter API internals documentation explains the structural part of
the automatic ordering in legacy mode (the filter config links and the
proxy lists holding the three orders, the segment boundaries of the
explicitly declared filters) and refers to the configuration manual for
the resulting orders, as it already does for the class orders of the new
mode.