CfC: Make definition of inbox and outbox more parallel

ActivityPub issue #289 
<https://github.com/w3c/activitypub/issues/289> describes a lack of 
parallelism between the definitions of "inbox" and "outbox" properties, 
giving rise to misunderstandings about the allowed values of those 
properties. The definitions are:

inbox/: *A reference*/*to an [ActivityStreams] OrderedCollection* 
comprised of all the messages received by the actor; see 5.2 Inbox.
outbox/: *An [ActivityStreams] OrderedCollection*/comprised of all the 
messages produced by the actor; see 5.1 Outbox.

I proposed an erratum on this list previously; Ben Goering noted 
correctly that it did not include a description of the actual error, 
which is preferred for W3C documents.

Based on his input, I updated theproposed erratum 
<https://www.w3.org/wiki/ActivityPub_errata/Proposed>to this text:

    In section 4.1 "Actor objects", the definition of "inbox" uses the
    imprecise term "reference" and is different from the definition of
    "outbox", giving the false impression that the range of the "inbox"
    property is different than that of "outbox". A possible correction
    is to make the definition of inbox parallel with that of outbox: "An
    OrderedCollection comprised of all the messages received by the
    actor; see 5.2 Inbox."

This is a call for consensus on adding this text to the errata for 
ActivityPub -- and then to the current editor's draft. Please comment 
here or on issue #289 if you object to this wording for the erratum. 
Absent objections in the next 14 calendar days (by end of day on 8 Aug 
in any timezone), I will add this text to the errata and the editor's draft.

Evan