They are like experimental features, but opt-in instead of opt-out. They
will allow us to gracefully remove language features. See #437
Change-Id: I9ca04cc48e6926750c4d622c2b229b25cc142c42
They are put in the manual separate pages under the new overarching
description of experimental features.
The settings page just lists the valid experimental feature names (so
people know what a valid setting entry looks like), with links to those
pages. It doesn't attempt to describe each experimental feature as that
is too much information for the configuration settings section.