Add missing changelogs and make existing ones match conventions

The conventions are not set in stone, however the changelog should not be a mixed bag of voices.

changelogs/application_service/newsfragments/1650.clarification

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+Change examples to use example.org instead of a real domain.

changelogs/client_server/newsfragments/1650.clarification

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+Change examples to use example.org instead of a real domain.

changelogs/client_server/newsfragments/1656.clarification

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+Clarify that ``state_default`` in ``m.room.power_levels`` always defaults to 50.

changelogs/client_server/newsfragments/1701.feature

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-Documented megolm session export format.
+Add megolm session export format.

changelogs/client_server/newsfragments/1744.clarification

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-Add missing status_msg to m.presence schema.
+Add missing ``status_msg`` to ``m.presence`` schema.

changelogs/client_server/newsfragments/1889.clarification

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-Add the missing `m.push_rules` event schema.
+Add the missing ``m.push_rules`` event schema.

changelogs/client_server/newsfragments/1891.clarification

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+Clarify how modern day local echo is meant to be solved by clients.

changelogs/client_server/newsfragments/1969.clarification

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+Fix various spelling mistakes throughout the specification.

changelogs/client_server/newsfragments/1975.clarification

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-Clarify that ``width`` and ``height`` are required parameters on ``/_matrix/media/r0/thumbnail/{serverName}/{mediaId}``. It is somewhat meaningless to request a thumbnail without specifying a desired size, and Synapse has never permitted such requests.
+Clarify that ``width`` and ``height`` are required parameters on ``/_matrix/media/r0/thumbnail/{serverName}/{mediaId}``.

changelogs/client_server/newsfragments/1999.clarification

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+Clarify how ``m.login.dummy`` can be used to disambiguate login flows.

changelogs/identity_service/newsfragments/1967.clarification

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+Fix route for ``/3pid/bind``.

changelogs/server_server/newsfragments/1650.clarification

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+Change examples to use example.org instead of a real domain.