DOC: lua: Document the filter instance declaration of Lua filters

DOC: lua: Document the filter instance declaration of Lua filters

The filter instance mode documentation claims that the supported filter
classes are documented in the section 9 of the configuration manual, but
the Lua filters are missing: they are not built-in classes, they are
registered by the Lua scripts with core.register_filter() and are
declared with the "lua" filter class. The Lua API documentation only
shows the legacy declaration of a Lua filter.

Document the "filter-config lua id <name>" declaration in the Lua API
documentation, next to the legacy one: the instance id selects the
registered filter to instantiate, the optional "enabled" token activates
the instance, and the other arguments are the same as for the legacy
declaration. Mention that contrary to the legacy "filter" directive, the
Lua filters may be declared in defaults sections with the new mode, to
be inherited by the proxies using them. Finally, point to the Lua API
documentation from the introduction of the section 9.