Clarify that room avatars cannot be encrypted

[Johennes]

Duplicating the info yaml isn't great but I think it'll be easier to wrangle than making the individual parameters reusable.

$ diff data/event-schemas/schema/core-event-schema/msgtype_infos/image_info.yaml data/event-schemas/schema/core-event-schema/msgtype_infos/avatar_info.yaml
1c1
< description: Metadata about an image.
---
> description: Metadata about an avatar image.
22d21
<       Only present if the thumbnail is unencrypted.
24,30d22
<   thumbnail_file:
<     description: |-
<       Information on the encrypted thumbnail file, as specified in
<       [End-to-end encryption](/client-server-api/#sending-encrypted-attachments).
<       Only present if the thumbnail is encrypted.
<     title: EncryptedFile
<     type: object
35c27
< title: ImageInfo
---
> title: AvatarInfo

Fixes: #562

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[richvdh]

LGTM!