telepathy-spec should have some markup that indicates that a property is useful in CreateChannel (and hypothetically, EnsureChannel) calls. Proposal (using TargetHandle as my example property):

<tp:requestable>
<p>If <a>TargetHandleType</a> is not Handle_Type_None, then either TargetHandle or <a>TargetID</a> must be given.</p>

<p>If <a>TargetHandleType</a> is omitted or Handle_Type_None, or <a>TargetID</a> is given, then TargetHandle must be omitted.</p>
</tp:requestable>

leading to output like:

<h3>Using this property in requests</h3>

This property can be used when requesting a channel.

<p>If <a>TargetHandleType</a> is not Handle_Type_None, then either TargetHandle or <a>TargetID</a> must be given.</p>

<p>If <a>TargetHandleType</a> is omitted or Handle_Type_None, or <a>TargetID</a> is given, then TargetHandle must be omitted.</p>