We should use real world examples instead of descriptions in field properties

For example, the PDU examples have `"ed25519:key_version:": "86BytesOfSignatureOfTheRedactedEvent"` which doesn't really show what the expected value could look like.

Instead, we should go through all the examples and make them match a real world example (replacing domain names with example.org). Ideally, those examples would have all or most of the possible characters present to show that things like `+` and `-` can be in some event IDs.

Start by locating the PDU examples and the field-property examples in the Matrix specification. Replace descriptive placeholders with realistic example.org values, including representative characters such as + and -, and finish when the examples demonstrate the expected value shapes throughout the specification.

Written by the indexing model from the issue text.