Document display_as parameter behavior for subject field

Updated Unstable API documentation to reflect that the display_as query parameter affects both body and subject fields in conversation responses. When set to plaintext, the subject field now returns plain text instead of HTML.

Related to intercom/intercom#438674

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Co-Authored-By: Claude <[email protected]>

Author: marcmlc

descriptions/0/api.intercom.io.yaml

@@ -6196,7 +6196,7 @@ paths:
       - name: display_as
         in: query
         required: false
-        description: Set to plaintext to retrieve conversation messages in plain text.
+        description: Set to plaintext to retrieve conversation messages in plain text. This affects both the body and subject fields.
         example: plaintext
         schema:
           type: string
@@ -6450,7 +6450,7 @@ paths:
       - name: display_as
         in: query
         required: false
-        description: Set to plaintext to retrieve conversation messages in plain text.
+        description: Set to plaintext to retrieve conversation messages in plain text. This affects both the body and subject fields.
         example: plaintext
         schema:
           type: string
@@ -18013,7 +18013,7 @@ components:
           example: operator_initiated
         subject:
           type: string
-          description: Optional. The message subject. For Twitter, this will show
+          description: Optional. The message subject. Returns HTML by default. When the display_as query parameter is set to plaintext, returns plain text instead. For Twitter, this will show
             a generic message regarding why the subject is obscured.
           example: ''
         body: