DOC: clarify the experimental status for certain features

DOC: clarify the experimental status for certain features

Certain features require "expose-experimental-directives" to be set in
the global section. Let's clarify that experimental featuers are only
maintained in best effort mode, may break during the stable cycle, and
are generally not maintained beyond the release of the next LTS branch
since it is extremely challenging, and early adopters are expected to
upgrade to benefit from improvements anyway.