Accept @equalsJeffH's rewrite of #sctn-attachment-from-transports

Co-authored-by: =JeffH <jdhodges@google.com>

index.bs

@@ -2430,39 +2430,25 @@ To remove the need to parse CBOR at all in many cases, {{AuthenticatorAttestatio
 Note: {{AuthenticatorAttestationResponse/getPublicKey()}} and {{AuthenticatorAttestationResponse/getAuthenticatorData()}} were only added in level two of this spec. [=[RPS]=] SHOULD use feature detection before using these functions by testing the value of `'getPublicKey' in AuthenticatorAttestationResponse.prototype`. [=[RPS]=] that require this function to exist may not interoperate with older user-agents.
 
 
-#### Determining Authenticator Attachment Options from Credential Transports #### {#sctn-attachments-from-transports}
+#### Determining Authenticator Attachment from Credential Transports #### {#sctn-attachments-from-transports}
 
-[INFORMATIVE]
-
-After creating a [=credential=], the [=[RP]=] may wish to know the [=authenticator attachment modality=] of the credential's [=managing authenticator=].
-This can be done by inspecting the result of calling {{getTransports()}}.
-
-If the result of {{getTransports()}}
+As a part of creating a {{PublicKeyCredential}} object, as the result of a  [=registration ceremony|registration=] or [=authentication ceremony=], the [=client platform=] determines the [=authenticator attachment modality=] and reports it in {{PublicKeyCredential/authenticatorAttachment}}. This section defines how to make that determination.
 
+If the client platform
 <dl class="switch">
-    : is [=list/empty=],
-    :: the [=authenticator attachment modality|attachment modality=] of the credential and its [=managing authenticator=] is unknown.
-
-    : [=list/contains=] only {{AuthenticatorTransport/internal}},
-    :: the credential is a [=platform credential=].
+    : cannot determine the {{transports}} used or the transport is not a member of {{AuthenticatorTransport}}, then
+    :: the [=authenticator attachment modality=] is unknown, and the value of {{PublicKeyCredential/authenticatorAttachment}} MUST be set to null.
 
-    : is not [=list/empty=] and does not [=list/contain=] {{AuthenticatorTransport/internal}},
-    :: the credential is a [=roaming credential=].
+    : used an {{AuthenticatorTransport/internal}} transport, then
+    :: {{PublicKeyCredential/authenticatorAttachment}} MUST be set to "platform".
 
-    : [=list/contains=] both {{AuthenticatorTransport/internal}} and other values,
-    :: the credential can be used as a [=platform credential=] under some circumstances and a [=roaming credential=] under some circumstances.
-        Both attachment options may or may not be available at the same time.
+    : used a {{transports|transport}} that is a member of {{transports}} other than {{AuthenticatorTransport/internal}},
+    :: {{PublicKeyCredential/authenticatorAttachment}} MUST be set to "cross-platform".
 
-        For example, a credential [=created on=] a mobile phone may be available as a [=platform credential=]
-        when the [=client device=] is that mobile phone,
-        and as a [=roaming credential=] when the [=client device=] is a laptop computer that can reach the mobile phone via Bluetooth.
-</dl>
-
-Note that an [=authenticator's=] [=authenticator attachment modality|attachment modality=] could change over time.
+Note: An [=authenticator's=] [=authenticator attachment modality|attachment modality=] could change over time.
 For example, a mobile phone might at one time only support [=platform attachment=]
 but later receive updates to support [=cross-platform attachment=] as well.
 
-
 ### Web Authentication Assertion (interface <dfn interface>AuthenticatorAssertionResponse</dfn>) ### {#iface-authenticatorassertionresponse}
 
 The {{AuthenticatorAssertionResponse}} interface represents an [=authenticator=]'s response to a client's request for