This PR adjusts the experimental module system to make `private` the
default visibility modifier in `module`s, introducing `public` as a new
modifier instead. `public section` can be used to revert the default for
an entire section, though this is more intended to ease gradual adoption
of the new semantics such as in `Init` (and soon `Std`) where they
should be replaced by a future decl-by-decl re-review of visibilities.
This PR makes the style of all `List` docstrings that appear in the
language reference consistent.
Relies on #7240 for links and example formatting.
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Co-authored-by: Kim Morrison <kim@tqft.net>
