Add note about leaveAdInterestGroup() and component ads (WICG#285)

We should also document the component ads, but that can come in another PR.

FLEDGE.md

@@ -102,7 +102,7 @@ navigator.joinAdInterestGroup(myGroup, 30 * kSecsPerDay);
 
 The browser will only allow the `joinAdInterestGroup()` operation with the permission of both the site being visited and the group's owner.  The site can allow or deny permission to any or all third parties via a `Permissions-Policy` (directive named "join-ad-interest-group"), where the default policy is to allow all in the top-level page and to deny all in cross-domain iframes.  The group's owner can indicate permission by `joinAdInterestGroup()` running in a page or iframe in the owner's domain, and can delegate that permission to any other domains via a list at a `.well-known` URL.  These can be combined, to allow a DSP to add a person to one of its interest groups based on publisher context, as discussed in [TERN](https://github.com/WICG/turtledove/blob/master/TERN.md#c-contextual-interest-groups) — provided the publisher's `Permissions-Policy` permits interest group additions by its SSP, and the DSP gives this SSP this ability.  If some permission is missing, `joinAdInterestGroup()` will raise an `Error` describing the reason for failure.
 
-There is a complementary API `navigator.leaveAdInterestGroup(myGroup)` which looks only at `myGroup.name` and `myGroup.owner`.  As a special case to support in-ad UIs, invoking `navigator.leaveAdInterestGroup()` from inside an ad that is being targeted at a particular interest group will cause the browser to leave that group, irrespective of permission policies.
+There is a complementary API `navigator.leaveAdInterestGroup(myGroup)` which looks only at `myGroup.name` and `myGroup.owner`.  As a special case to support in-ad UIs, invoking `navigator.leaveAdInterestGroup()` from inside an ad that is being targeted at a particular interest group will cause the browser to leave that group, irrespective of permission policies. Note that leaving without arguments isn't supported inside a component ad frame.
 
 The browser will remain in an interest group for only a limited amount of time.  The duration is specified in the call to `joinAdInterestGroup()`, and will be capped at 30 days.  This can be extended by calling `joinAdInterestGroup()` again later, with the same group name and owner.  Successive calls to `joinAdInterestGroup()` will overwrite the previously-stored values for any interest group properties, like the group's `userBiddingSignal` or list of ads.