The "connection.stable-id" supports placeholders like "${CONNECTION}" or
"${DEVICE}".

The stable-id can also be specified in global connection defaults in
NetworkManager.conf, by leaving it unset in the profile. Global
connection defaults always follow the pattern, that they correspond to a
per-profile property, and only when the per-profile value indicates a
special default/unset value, the global connection default is consulted.
Finally, if the global connection default is also not configured in
NetworkManager.conf, a built-in default is used (which may not be
constant either, for example ipv6.ip6-privacy's built-in default depends
on a sysctl value).

In any case, every possible configuration that can be achieved should be
configurable both per-profile and via global connection default. That
was not given for the stable-id, because the built-in default generated
an ID in a way that could not be explicitly expressed otherwise.

So you could not:
- explicitly set the per-profile value to the built-in default, to avoid
  that the global-connection-default overwrites it.
- explicitly set the global-connection-default to the built-in default,
  to avoid that a lower priority [connection*] section overwrites the
  stable-id again.

Fix that inconsistency to make it possible to explicitly set the
built-in default.

Change behavior for literally "default${CONNECTION}" and make it behave
as the built-in default. Also document that the built-in default has that
value.

It's unlikely that this breaks an existing configuration, but of course,
if any user configured "connection.stable-id=default${CONNECTION}", then
the behavior changes for them.